ARM: dts: msm: add DT node for GPIO keys on MSM8937

Add gpio based keys device node to MSM8937. The supported
keys are VOLUME UP, CAMERA SNAPSHOT, CAMERA FOCUS for both
CDP and MTP and HOME KEY only for CDP.

Change-Id: I4d18d26ebd7f811404ab2fef2abd8133a56cd572
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>