dt-bindings: arm: at91: Document Kizbox2-2 board binding

Document devicetree's binding for the Kizbox2-2 board of
Overkiz SAS based on SAMA5D31 SoC.

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191105212234.22999-1-kamel.bouhara@bootlin.com


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