dt-bindings: Add bindings for SPI NAND devices

Add bindings for SPI NAND chips.
Signed-off-by: NBoris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>

Documentation/devicetree/bindings/mtd/spi-nand.txt

+SPI NAND flash
+
+Required properties:
+- compatible: should be "spi-nand"
+- reg: should encode the chip-select line used to access the NAND chip