mfd: pm8038-core: Add support for PMIC thermal alarm

Add pm8xxx-tm subdevice into pm8038-core.  The pm8xxx-tm device
can be used to access the real-time PMIC die temperature.  It
can also be used to trigger an interrupt when critical
temperature thresholds are reached.

Change-Id: I4b8bda1d51cafaa6b77408da4c183bdb2dc4327a
Signed-off-by: David Collins <collinsd@codeaurora.org>