power: pm8921-bms: Enable batt alarm

Set the batt alarm to wake up the system if the
battery voltage falls below a threshold. This is to
make sure that the device does not shutdown abruptly
without any notification.

Hold the low voltage wakelock to prevent the device
from going to sleep immediately after the low voltage
notification.

Change-Id: Ifaeeeb27bd85f9c95f026ac3565681432e247b02
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>