platform: qpnp-power-on: add support for S3 source

Stage 3 (S3) reset is designed to recover the device
from unexpected hang-ups for the devices having
non-removable batteries. The source for S3
trigger is supplied from device tree data.

CRs-fixed: 606366
Change-Id: I32cc3dec4982b9ecade44663b5e02c71dec184d7
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>