dt-bindings: iio: adc: Add PMIC7 VADC bindings

Add documentation for PMIC7 VADC peripheral.
PMIC7 ADC has support for clients to read voltage and
current on supported channels connected to the PMIC.
Clients include reading voltage phone power and on board system
thermistors for thermal management. ADCs on certain PMICs have
support to read battery current.

Change-Id: Ieb8ad5779b078dd77838c498b3a4e0bbcf907df8