dt-bindings: Add bindings for SPI NAND devices

Add bindings for SPI NAND chips.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>

doc/device-tree-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