arm/dt: msm8974: Break out gpios into board specific files

The file msm8974-gpio.dtsi is common between multiple msm8974
based boards. Since the usecase of gpios is board-specific, move
the pin definitions to all current msm8974 board dts files.

Change-Id: I82b097f8c2c9d8e4d21a35b39571059b6d0208bf
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>