ARM: dts: at91: sama5d2: add adc device

Add the ADC device, and remove the adc_op_clk which is useless since the
adc sampling frequency is configured with sysfs.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>