iio: adc: at91-sama5d2_adc: add support for sama7g5 device

Add support to sama7g5 ADC which is similar with sama5d2/sam9x60 device.
Differences are highlighted by compatible.
Main differences include 16 channels instead of 12 and missing
resistive touchscreen.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20210901123013.329792-8-eugen.hristev@microchip.com


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