mfd: pm8xxx-misc: Add API to read PMIC registers

This API reads the PMIC registers at runtime and can be called
from process/interrupt context. It is currently enabled for PM8921.

Change-Id: I63a80293e37725569d075de814f27f67382976ca
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>
Merge "mfd: pm8xxx-misc: Add API to read PMIC registers"