dt-bindings: mtd: nand-chip: Create a NAND chip description

Move the NAND chip description out of the NAND controller file. Indeed,
a subsequent part of the properties supported by a raw NAND chip are
also supported by SPI-NAND chips. So let's create a generic NAND chip
description which will be pulled by nand-controller.yaml and later by
spi-nand.yaml as well.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-5-miquel.raynal@bootlin.com