iio: adc: Add support for PMIC7 VADC

Add support for VADC driver on PMIC7 for Lahaina
architecture. The main change for VADC from PMIC5 to
PMIC7 is that all SW communication to ADC goes through
PMK8350, which communicates with other PMICs through PBS,
using PMIC SID number to distinguish between different PMICs.

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