hwmon: qpnp-adc: Add PMIC QPNP VADC Driver

The QPNP VADC driver supports the User Bank Peripheral of
the voltage ADC(VADC).

VADC is a 15 bit ADC that measures signals through the
Main analog multiplexer (AMUX) and PREMUX. The driver
arbitrates the request to issue ADC read requests.

VADC driver includes support for the conversion sequencer.
The conversion sequencer is a HW triggered signal to start
ADC measurement on trigger events for PA ON,
camera flash and TX threshold.

The AMUX supports external pull-ups simultaneously. Clients
can select the appropriate AMUX input channel to measure the
ADC for the intended pull up configuration.

Change-Id: I8886968ccec54ad03334b113b4516d4d200e0da8
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>