iio: adc: Add PMIC VADC channel support

Add scaling support for VADC channels on PM8998.
Add PMIC5 ADC voltage channel support and scaling
for PM855, PM855B and PM855l.

Change-Id: I3bba20977da60ae8a02265ea1e66530331c53596
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>