ARM: dts: at91: sama7g5ek: enable ADC on the board

The ADC controller on the board is fed by a 2.5V reference voltage.
By default the channels #14 and #15 are dedicated to analog input
(marked AN on the board), on the connectors mikrobus1 and mikrobus2.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210901123013.329792-11-eugen.hristev@microchip.com