iio: adc: spmi-vadc: Update function for generic voltage conversion

Several channels are supported in ADC of PMIC which can be used to
measure voltage, temperature, current etc., Hardware provides
readings for all channels in adc code. That adc code needs to be
converted to voltage. Logic for conversion of adc code to voltage
is common for all ADC channels(voltage, temperature, current
.,etc). Implement separate function for generic conversion logic.

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