dt-bindings: ARM: at91: document exegin q5xr5 board

Document Exegin Q5xR5 board.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210816064416.1630674-7-claudiu.beznea@microchip.com