hwmon: qpnp-adc: Enable VADC_HC peripheral

Add support to allow clients to perform single
measurement requests on VADC_HC peripheral.
Channels include reading vph_pwr input, PMIC
die temperature, coin cell input voltage,
battery input and system thermistors.

The peripheral configures individual channel
properties such as the calibration type, hardware
settling delay, fast averaging and decimation ratio
associated with the channel when the individual
channel measurement request is issued. The client
receives the raw ADC code and the physical scaled
result associated with the channel.

Change-Id: I73bfdb05b7561efa4b3d7c8d2fa7f8d0df5ea48e
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>