iio: adc: Add support for PMIC7 VADC

The VADC architecture on PMIC7 is changed as compared to PMIC5. The
major change from PMIC5 is that all SW communication to ADC goes through
PMK8350, which communicates with other PMICs through PBS when the VADC
on PMK8350 works in master mode. The SID register is used to identify the
PMICs with which the PBS needs to communicate. Add support for the same.

Change-Id: If5c651e01be9363a61d1bf546e0b6d82a023904c
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>