iio: adc: spmi-vadc: Changes to support different scaling

Several ADC channels are supported in PMIC which can be used to
measure voltage, temperature, current etc. Different scaling can be
applied on the obtained voltage to report in physical units. Scaling
functionality can be different per channel. Add scaling support per
channel. Every channel present in adc has an unique conversion formula
for obtained voltage. Add support to report in Raw as well as in
processed format. Scaling is applied when processed read is requested
and is not applied when a Raw read is requested.

Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>