iio: adc: add RZ/T2H / RZ/N2H ADC driver

iio: adc: add RZ/T2H / RZ/N2H ADC driver

Add support for the A/D 12-Bit successive approximation converters found
in the Renesas RZ/T2H (R9A09G077) and RZ/N2H (R9A09G087) SoCs.

RZ/T2H has two ADCs with 4 channels and one with 6.
RZ/N2H has two ADCs with 4 channels and one with 15.

Conversions can be performed in single or continuous mode. Result of the
conversion is stored in a 16-bit data register corresponding to each
channel.

The conversions can be started by a software trigger, a synchronous
trigger (from MTU or from ELC) or an asynchronous external trigger (from
ADTRGn# pin).

Only single mode with software trigger is supported for now.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>