dt-bindings: arm: add sam9x60-ek board

Add documentation for SAM9X60-EK board.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-16-git-send-email-claudiu.beznea@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>