hwmon: pm8921-adc: Add sysfs interface

-Add sysfs interface to read PM8921 HK/XOADC
and thermistor temperature measurements mapped
through MPP's.
-Move PM8921 ADC from directory /drivers/mfd to
/drivers/hwmon for userspace clients to read ADC
through hwmon.

CRs-Fixed: 302365
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>

Conflicts:

	drivers/mfd/Kconfig