dt-bindings: iio: update pm8350b ADC channel list

Add PM8350B_ADC7_ICHG_FB channel to read charging current for
PM8350B. Also, update qcom,spmi-vadc.h to add ADC7_ICHG_FB and
other relevant channels that are needed for reading input and
charging current for SMB139x charger pumps along with scale
function indices.

Change-Id: I4a1b53f328e1b394063cbf951f6ff978eb96d21e
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>