dt-bindings: arm: at91: Document Kizboxmini and Smartkiz boards binding

Document devicetree's bindings for the Overkiz's Kizbox Mini and
Smartkiz boards, based on a SAM9G25 Atmel SoC.

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191220103835.160154-1-kamel.bouhara@bootlin.com


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