mfd: pm8xxx: Add PM8xxx battery alarm driver

Add a driver to control the battery alarm module of PMIC PM8xxx
devices.  This module uses a pair of comparators to determine
when battery under and over-voltage take place.  A wakeup
interrupt is triggered in these cases which can then run any
notifiers which have been registered.  Also add APIs to
configure the threshold voltages and the frequency at which the
hardware checks the state of the battery voltage.

Change-Id: Id0b82f9090b29ce743b5e0faac17853c94111771
Signed-off-by: David Collins <collinsd@codeaurora.org>