thermal: pm8xxx-tm: Turn off the temperature alarm during shutdown

The temperature alarm consumes additional current if it remains
turned on when the device is off. Turn off the alarm in the
shutdown hook, as it not required when the device is off.

CRs-Fixed: 474628
Change-Id: Ibfaee6b74aeebcb530ae7375f623955f80df148e
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>