iio: adc: at91_adc: rework resolution selection

iio: adc: at91_adc: rework resolution selection

Move the possible resolution values back to the driver. This removes the
atmel,adc-res and atmel,adc-res-names properties, leaving only
atmel,adc-use-res. As atmel,adc-res-names had to contain "lowres" and
"highres", those where already the only allowed values for
atmel,adc-use-res.

Also introduce a new compatible string for the sama5d3 as this is the only
one with a different resolution. Also it doesn't even have the LOWRES
bit.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20201128222818.1910764-3-alexandre.belloni@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>