dt-bindings: mtd: sunxi-nand: Add YAML schemas

Switch the DT binding to a YAML schema to enable the DT validation.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>