iio: qcom-spmi-adc5: Update ADC5 Gen3 support for PM5100

Add scaling functions for batt_therm, batt_id and usb_in_i ADC channels
for PM5100. Also add usb_sns_v_div_16 and vin_div_16 channels to list of
supported channels and correct default prescaling value for vbat_sns
channel and full scale ADC current.

Change-Id: I86b86c319bff6df583efab8cc4c877218a79fe56
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>