dt-bindings: arm: at91: Document Kizbox3 HS board binding

Document devicetree binding of SAMA5D27 Kizbox3 HS board from Overkiz
SAS.

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Link: https://lore.kernel.org/r/20191011125022.16329-3-kamel.bouhara@bootlin.com


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>