msm: qpnp-power-on: Add API to detect warm reset

Add an API to allow consumers to know whether the PMIC went through
a warm or cold boot. This currently only checks the PON device that
allows resets.

Change-Id: Id145ec62bbdf0e4988476f2c2084f92c3f588209
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>