dt-bindings: iio: Add PMIC7 ADC support

Add definitions for ADC channels and scaling function IDs
for PMIC7 ADC drivers. Add virtual channel definitions per
PMIC, to be used by ADC clients for PMIC7.

Change-Id: If857612a8087b928901dc08327aea3a8f4df7b01
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>