ARM: dts: at91: sama5d2_xplained: enable ADTRG pin

Enable pinctrl for ADTRG pin (PD31) for ADC hardware trigger support.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>