mfd: pm8821-core: Add support for PMIC thermal alarm

Add a pm8xxx-tm subdevice into pm8821-core.  The pm8xxx-tm device
can be used to receive an interrupt when critical temperature
thresholds are reached.  Since PM8821 does not have an ADC, it is
not possible to read a real time PM8821 die temperature using
this pm8xxx-tm device.

Change-Id: I3720c79a0546e8bc69044166f70dd4c313e84daa
Signed-off-by: David Collins <collinsd@codeaurora.org>