iio: adc: add support for pac1921

Add support for Microchip PAC1921 Power/Current monitor.

Implemented features:
* capture of bus voltage, sense voltage, current and power measurements
  in free-run integration mode
* support for both raw and triggered buffer reading
* support for overflow events
* scale attributes to control voltage and current gains
* oversampling ratio attribute to control the number of integration
  samples
* sampling rate attribute that reflects the integration period
* userspace attribute and DT parameter to control shunt resistor
* simple power management support

Limitations:
* operation mode fixed to free-run integration
* READ/INT pin and OUT pin not supported
* no controls for measurement resolutions and filters

Signed-off-by: Matteo Martelli <matteomartelli3@gmail.com>
Link: https://patch.msgid.link/20240724-iio-pac1921-v4-3-723698e903a3@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>