dt-bindings: mtd: spi-nand: Convert spi-nand description file to yaml

Let's get rid of spi-nand.txt by converting it to yaml schema. While at
converting this file, let's actually pull all the generic properties
from nand-chip.yaml which might apply to a SPI-NAND chip.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-6-miquel.raynal@bootlin.com