dt-bindings: mtd: spi-nor: Convert to DT schema format

Convert the SPI-NOR binding to DT schema format. Like other memory chips,
the compatible strings are a mess with vendor prefixes not being used
consistently and some compatibles not documented. The resulting schema
passes on 'compatible' checks for most in tree users with the exception
of some oddballs.

I dropped the 'm25p.*-nonjedec' compatible strings as these don't appear
to be used anywhere.

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210202175340.3902494-1-robh@kernel.org