iio: adc: meson-saradc: add support for Meson AXG

Add the SAR ADC driver for the Amlogic Meson-AXG SoC.

Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>