[201811][platform-common] add platform-common 201811 branch (sonic-ne…

…t#2743)

- Cherry-picked one change:

Submodule src/sonic-platform-common d4bf78c..42119e1:
  > remove gang port parsing (#24)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

.gitmodules

@@ -45,6 +45,7 @@
 [submodule "src/sonic-platform-common"]
  path = src/sonic-platform-common
  url = https://github.com/Azure/sonic-platform-common
+ branch = 201811
 [submodule "src/sonic-platform-daemons"]
  path = src/sonic-platform-daemons
  url = https://github.com/Azure/sonic-platform-daemons