iio: adc: Add support for QCOM PMIC5 Gen3 ADC

The ADC architecture on PMIC5 Gen3 is similar to that on PMIC5 Gen2,
with all SW communication to ADC going through PMK8550 which
communicates with other PMICs through PBS.

One major difference is that the register interface used here is that
of an SDAM (Shared Direct Access Memory) peripheral present on PMK8550.
There may be more than one SDAM used for ADC5 Gen3 and each has eight
channels, which may be used for either immediate reads (same functionality
as previous PMIC5 and PMIC5 Gen2 ADC peripherals) or recurring measurements
(same as ADC_TM functionality).

By convention, we reserve the first channel of the first SDAM for all
immediate reads and use the remaining channels across all SDAMs for
ADC_TM monitoring functionality.

Add support for PMIC5 Gen3 ADC driver for immediate read functionality.
ADC_TM is implemented as an auxiliary thermal driver under this ADC
driver.

Signed-off-by: Jishnu Prakash <jishnu.prakash@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>