thermal: pm8xxx-tm: Configure temperature alarm for always-on operation

Change the register initialization for the PMIC temperature alarm
module so that it is always on.  This ensures that die
temperature monitoring is active even if CXO is disabled (i.e.
when sleep_b is low).  This is necessary since CXO can be
disabled while the system is still heavily loaded.  Also, using
the alway-on instead of PWM-enabled configuration ensures that
the die temperature can be measured by the PMIC ADC without
reconfiguring the temperature alarm module first.

Change-Id: I4a76c959c81f222fbeca08b6c98b1e4b23653b4e
Signed-off-by: David Collins <collinsd@codeaurora.org>