msm: qpnp-power-on: print the power off reason

The PMIC power off reason is often very useful for debugging. By
printing the power off reason, testers can tell whether a shutdown
was a normal controlled shutdown by the MSM or a unexpected PMIC
shutdown.

Add a single log message per PMIC to print out the power off reason
and a short explanation of the likely cause.

Change-Id: I438e5915f6ac7372af6e39e5dff7a3709623b763
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>