Inherit from msm8974-common

* msm8974-common then inherits from qcom-common.
* This doesn't change anything currently, just adds one more
  common tree in between - however, later on, we can start moving
  things to this new tree as needed, from both shinano-common
  and qcom-common

Change-Id: Ib0a5bca4569501e8504ae18b04c98f59130fe4be