mfd: pmic8038-core: Add HK/XOADC device

HouseKeeping ADC (HKADC) on the PMIC is used by clients
to measure voltage, current and temperature. Clients
include the Battery, Charger and Thermal clients that
require reading the ADC in real time. This also
supports reading the ADC channels that are mapped through
the PMIC MPP's such as the PA THERM channel.

Change-Id: I3c4fd9d368550c1cca168ea097e0d457d01d695e
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>