iio: qcom-spmi-adc5: add support to read current channels

Add support to read current channels on PM8350B and SMB139x
charge pumps.

IIN_FB - Input current drawn by main charger (PM8350B)
ICHG_FB - Charging current from main charger (PM8350B)
IIN_SMB - Input current drawn by one of SMB139x
ICHG_SMB - Charging current from one of SMB139x

Since, most of these channels needs prescale ratios post reading
ADC code and before scaling, add the necessary prescale ratios.

Change-Id: Id9b866f684d574813904a5fd8e658f59b1d7bcbf
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>