ARM: dts: msm: add keys device node

Add gpio based keys device node to msm8994 cdp.
The supported keys are VOLUME UP, CAMERA SNAPSHOT and
CAMERA FOCUS.

Change-Id: Ib45921d01d344c8694dee6c0208cbfaab1b5377f
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>