[mellanox]: Support warm reboot on MSN4700 (sonic-net#4910)

Author: stephenxs

device/mellanox/x86_64-mlnx_msn4700-r0/ACS-MSN4700/sai_4700.xml

@@ -5,6 +5,9 @@
         <!-- Device MAC address  -->
         <device-mac-address>00:02:03:04:05:00</device-mac-address>
 
+        <!-- ISSU enabled -->
+        <issu-enabled>1</issu-enabled>
+
         <!-- Number of ports in the following port list -->
         <number-of-physical-ports>32</number-of-physical-ports>