mfd: pm8921-adc: Fix scaling and PM8921 MPP ADC read

-Modify the existing PM8921 ADC read api to include
reading ADC mapped to devices on the MPP.
-Modify scaling function to support XOTHERM and PATHERM
on PM8921.
-Remove post scaling functions for external charger
as its not used on PM8921.

CRs-Fixed: 307164
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
3 files changed