iio: adc5: Add channels and add batt_therm support for SM6150

Add channel entries in channel list for SM6150.
In addition, add scaling support to read batt_therm for
for pullup resistances of 30K, 100K and 400K.

Change-Id: I577e382ebd74498a83ce0754f8dbc7b32a4f8513
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>