iio: adc: Add PMIC7 ADC bindings

Add documentation for PMIC7 ADC peripheral.
For the PMIC7-type PMICs, ADC peripheral is present in HW for the
following PMICs: PMK8350, PM8350, PM8350b, PMR735a and PMR735b.
Of these, only the ADC peripheral on PMK8350 is exposed directly to SW.
If SW needs to communicate with ADCs on other PMICs, it specifies the
PMIC to PMK8350 through the newly added SID register and communication
between PMK8350 ADC and other PMIC ADCs is carried out through
PBS(Programmable Boot Sequence) at the firmware level.

In addition, add definitions for ADC channels and virtual channel
definitions (combination of ADC channel number and PMIC SID number)
per PMIC, to be used by ADC clients for PMIC7.

Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>