ARM: dts: msm: add gpio_key devices for buttons on sdmshrike MTP and CDP

Add pinctrl configurations for home and volume up buttons which
utilize PM855_1 GPIOs 1 and 6 respectively.  Add gpio_key devices
for these buttons on SDMSHRIKE MTP and CDP boards.

Change-Id: I14c15544ce41444b929d7b8128f47deea221a990
Signed-off-by: David Collins <collinsd@codeaurora.org>