dt-bindings: iio: adc: Add support for QCOM PMIC5 Gen3 ADC

dt-bindings: iio: adc: Add support for QCOM PMIC5 Gen3 ADC

For the PMIC5-Gen3 type PMICs, ADC peripheral is present in HW for the
following PMICs: PMK8550, PM8550, PM8550B and PM8550VX PMICs.

It is similar to PMIC5-Gen2, with SW communication to ADCs on all PMICs
going through PBS(Programmable Boot Sequence) firmware through a single
register interface. This interface is implemented on SDAM (Shared
Direct Access Memory) peripherals on the master PMIC PMK8550 rather
than a dedicated ADC peripheral.

Add documentation for PMIC5 Gen3 ADC and update SPMI PMIC bindings to
allow ADC5 Gen3 as adc@ subnode.

Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jishnu Prakash <jishnu.prakash@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>