iio: adc: Update reading USB_IN_V channel

Update USB_IN channel read sequence. This channel read
sequence is done to prevent over voltage in charger.
As part of the read sequence update the calibration
measurement needs to be updated and requires accessing
the CAL peripheral. The read sequence is followed
if the optional property to read the CAL peripheral
is available for the PMIC that supports reading
the USB_IN channel.

Change-Id: I31fa5b3340fdd744e192e3ec09061d1915d12682
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>