[Mellanox]Correct reboot cause when reboot via power cycle (#3597)

* [sonic_platform]remove the handling of reset_sw_reset which indicates rebooted by software.

* [sonic_platform]Check "reset_sw_reset"
Also check reboot cause file "reset_sw_reset" which indicates the system was rebooted due to software requesting.

Author: stephenxs

platform/mellanox/mlnx-platform-api/sonic_platform/chassis.py

@@ -319,11 +319,11 @@ def initialize_reboot_cause(self):
             'reset_comex_wd'            :   "Reset requested from ComEx",
             'reset_from_asic'           :   "Reset requested from ASIC",
             'reset_reload_bios'         :   "Reset caused by BIOS reload",
-            'reset_sw_reset'            :   "Software reset",
             'reset_hotswap_or_halt'     :   "Reset caused by hotswap or halt",
             'reset_from_comex'          :   "Reset from ComEx",
             'reset_voltmon_upgrade_fail':   "Reset due to voltage monitor devices upgrade failure"
         }
+        self.reboot_by_software = 'reset_sw_reset'
         self.reboot_cause_initialized = True
 
 
@@ -350,6 +350,11 @@ def get_reboot_cause(self):
             if self._verify_reboot_cause(reset_file):
                 return self.REBOOT_CAUSE_HARDWARE_OTHER, reset_cause
 
+        if self._verify_reboot_cause(self.reboot_by_software):
+            logger.log_info("Hardware reboot cause: the system was rebooted due to software requesting")
+        else:
+            logger.log_info("Hardware reboot cause: no hardware reboot cause found")
+
         return self.REBOOT_CAUSE_NON_HARDWARE, ''