hwmon: pm8xxx-adc: Vote for a PMIC clock buffer

Incorrect PMIC_THERM/XO_THERM readings are seen when
the XO clock is disabled.

The internal PMIC therm relies on the PMIC band gap
reference which in turn is enabled when any of the
PMIC clock buffers are enabled. The XO_THERM relies
on the VREF_XO as its reference source which gets
enabled when any of the PMIC clock buffers are
switched on. Therefore vote for a clock buffer when
a PMIC_THERM/XO_THERM is requested.

CRs-Fixed: 392602
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Change-Id: I2165f5d52b537c6b589a22a20feede61e88b5ec4