[DeviceBase] Added hardware revision number to generic device proper…

…ties (sonic-net#184)

Added a `get_revision()` method to the generic platform device class which returns the hardware revision number of the device as a string.

sonic_platform_base/device_base.py

@@ -55,6 +55,15 @@ def get_serial(self):
         """
         raise NotImplementedError
 
+    def get_revision(self):
+        """
+        Retrieves the hardware revision of the device
+
+        Returns:
+            string: Revision value of device
+        """
+        raise NotImplementedError
+
     def get_status(self):
         """
         Retrieves the operational status of the device