arm/dt: msm8974: Add device and GPIO configuration for GPIO based keys

Add support for 3 keys connected on PM8941 GPIOs and specify the
required GPIO configuration for these keys.

The supported keys are -
camera snapshot - PM8941 GPIO 3
camera focus - PM8941 GPIO 4
volume up - PM8941 GPIO 5

Change-Id: Icdc6f31fa5055c7e9b86a541e618cd421a0dcd2a
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>