[Arista]: Update Arista7060X6-PE 256x200G mmu configuration. (sonic-net#19570)

Why I did it
This PR updates the MMU related configurations on Arista 7060X6-PE device with 256x200G breakout.

Work item tracking
Microsoft ADO (number only): 28707303
How I did it
This PR updates 3 things:

- Updated bcm file for optimal MMU settings.
- Updated buffer defaults to accommodate the TH5 architecture with 1 ingress pool + 1 egress pool and updated to optimal value.
- Updated PG lookups for buffer setups.

How to verify it

Tested with sonic-mgmt tests with xoff/xon tests with updated QoS parameter: https://github.com/sonic-net/sonic-mgmt/pull/13656/files
Local ixia test is passing in lab.
Both verified using 202311 branch for backporting.

Author: r12f

device/arista/x86_64-arista_7060x6_64pe/Arista-7060X6-64PE-256x200G/BALANCED

@@ -0,0 +1,36 @@
+{%- set default_cable = '5m' %}
+
+{%- include 'buffer_ports.j2' %}
+
+{%- macro generate_buffer_pool_and_profiles() %}
+    "BUFFER_POOL": {
+        "ingress_lossless_pool": {
+            "size": "165660324",
+            "type": "ingress",
+            "mode": "dynamic",
+            "xoff": "29520896"
+        },
+        "egress_lossless_pool": {
+            "size": "165660324",
+            "type": "egress",
+            "mode": "dynamic"
+        }
+    },
+    "BUFFER_PROFILE": {
+        "ingress_lossy_profile": {
+            "pool": "ingress_lossless_pool",
+            "size": "0",
+            "static_th": "167546528"
+        },
+        "egress_lossy_profile": {
+            "pool": "egress_lossless_pool",
+            "size": "1778",
+            "dynamic_th": "0"
+        },
+        "egress_lossless_profile": {
+            "pool": "egress_lossless_pool",
+            "size": "0",
+            "static_th": "167546528"
+        }
+    },
+{%- endmacro %}

device/arista/x86_64-arista_7060x6_64pe/Arista-7060X6-64PE-256x200G/BALANCED/buffers_defaults_t0.j2

@@ -0,0 +1,36 @@
+{%- set default_cable = '5m' %}
+
+{%- include 'buffer_ports.j2' %}
+
+{%- macro generate_buffer_pool_and_profiles() %}
+    "BUFFER_POOL": {
+        "ingress_lossless_pool": {
+            "size": "121964196",
+            "type": "ingress",
+            "mode": "dynamic",
+            "xoff": "29520896"
+        },
+        "egress_lossless_pool": {
+            "size": "121964196",
+            "type": "egress",
+            "mode": "static"
+        }
+    },
+    "BUFFER_PROFILE": {
+        "ingress_lossy_profile": {
+            "pool": "ingress_lossless_pool",
+            "size": "0",
+            "static_th": "167546528"
+        },
+        "egress_lossy_profile": {
+            "pool": "egress_lossless_pool",
+            "size": "1778",
+            "dynamic_th": "1"
+        },
+        "egress_lossless_profile": {
+            "pool": "egress_lossless_pool",
+            "size": "0",
+            "static_th": "125995684"
+        }
+    },
+{%- endmacro %}

device/arista/x86_64-arista_7060x6_64pe/Arista-7060X6-64PE-256x200G/BALANCED/buffers_defaults_t1.j2

@@ -0,0 +1,23 @@
+# PG lossless profiles.
+# speed cable size    xon  xoff    threshold xon_offset
+  10000  5m   1248    2288 35776   0         2288
+  25000  5m   1248    2288 53248   0         2288
+  40000  5m   1248    2288 66560   0         2288
+  50000  5m   1248    2288 90272   0         2288
+ 100000  5m   18796   3556 300990  -2        3556
+ 200000  5m   18796   3556 300990  -2        3556
+ 400000  5m   18796   3556 300990  -2        3556
+  10000  40m  1248    2288 37024   0         2288
+  25000  40m  1248    2288 53248   0         2288
+  40000  40m  1248    2288 71552   0         2288
+  50000  40m  1248    2288 96096   0         2288
+ 100000  40m  18796   3556 300990  -2        3556
+ 200000  40m  18796   3556 300990  -2        3556
+ 400000  40m  18796   3556 300990  -2        3556
+  10000  300m 1248    2288 46176   0         2288
+  25000  300m 1248    2288 79040   0         2288
+  40000  300m 1248    2288 108160  0         2288
+  50000  300m 1248    2288 141856  0         2288
+ 100000  300m 18796   3556 300990  -2        3556
+ 200000  300m 18796   3556 300990  -2        3556
+ 400000  300m 18796   3556 300990  -2        3556