ARM: dts: msm: add gpio_key devices for buttons on SDM855 MTP, CDP and QRD

Add pinctrl configurations for home and volume up buttons which
utilize PM855 GPIOs 1 and 6 respectively.  Add gpio_key devices
for these buttons on SDM855 MTP, CDP and QRD targets.

Change-Id: Ifc5fba0d068401e2c020dd6462a5bf10889ec30a
Signed-off-by: David Collins <collinsd@codeaurora.org>