ARM: dts: at91: sama5d27_som1_ek: enable ADC node

Enable the ADC for AN pins on Mikrobus1 and Mikrobus2 on the board.
These correspond to channels AD6 and AD7 in the controller.

 # cat /sys/bus/iio/devices/iio\:device0/in_voltage6_raw
 240
 # cat /sys/bus/iio/devices/iio\:device0/in_voltage7_raw
 16380

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210622101742.14535-1-eugen.hristev@microchip.com