ARM: dts: at91: sama5d2_xplained: use pin macros instead of numbers

Use pin macros instead of magic numbers to ease interpretation.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>