iio: adc: Add support for PMIC7 ADC

The ADC architecture on PMIC7 is changed as compared to PMIC5. The
major change from PMIC5 is that all SW communication to ADC goes through
PMK8350, which communicates with other PMICs through PBS when the ADC
on PMK8350 works in master mode. The SID register is used to identify the
PMICs with which the PBS needs to communicate. Add support for the same.

Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>