ARM: dts: at91: sama5d3: use proper ADC compatible

The ADC is different from the at91sam9x5 ADC. Not only it doesn't have the
same resolution but it even has only one and the LOWRES bit doesn't exist.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20201128222818.1910764-9-alexandre.belloni@bootlin.com