ARM: dts: msm: Fix GPIO controller phandles on 8994

The device definitions in the board-specific DTSI files for
8994 Interposer targets have references to the 8974 GPIO
controller node. Refactor these to point to the the 8994
GPIO controller node, and create overrides for interposer
backward compatibility.

Change-Id: I563b36e5ca43b41ef8b2547236608c55347e0a27
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>