platform: qpnp-power-on: Store device restart reason in PMIC register

Use the spare register in PMIC power-on peripheral to store the device
restart reason. The reason can be retained whenever executing a soft/hard
PMIC reset.

Change-Id: I5f44b82ccc0a1780f09157f81a9dc0c257c7099b
Signed-off-by: Lijuan Gao <lijuang@codeaurora.org>
Signed-off-by: Kishan Kumar <kishank@codeaurora.org>