klte-common: add PowerHAL extension for gpio-keys.

* This should have been here all along. This is how its done in hlte. we
actually had this in at one point but it got removed by mistake during
bringup.

Change-Id: Ib295b7fc1ffc079a299187cad32d59fc2ff9dce8
(cherry picked from commit 21fc2c0b7736b68100b76a14a4741a3f7163d4f5)