hwmon: qpnp-adc: Add pinctrl property

PMIC GPIO can be configured as an analog input for
reading ADC voltage on supported channels. The GPIO
has to be configured in high impedance state and left
off when the pin is used with the VADC.

Change-Id: Ieb1a6771bbc7f0023c3525083499a6ef841f4b88
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>