hwmon: qpnp-adc: Add support for USB_IN_V channel on PMI632

USB_IN_V cannot be measured similarly to other channels due to
its low pre-scaling factor, which can cause its reading to be
distorted by residual voltage from previous ADC measurements.
Add support to lower calibration frequency and add delay before
measurement to ensure accurate reading.

Change-Id: If35f5c692a996d6f732038fabd5e1205525dbc03
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>