iio: adc: Add support to read ADC5 current channels

Add scaling function required to read PMIC5 current ADC channels
so that channel 0xA5 (ADC5_PARALLEL_ISENSE) can use that.

Change-Id: I55ede0c736c6f350df8ac92ea8661db4129da13c
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>