dt-bindings: mtd: spi-nor: clarify the need for spi-nor compatibles

Most SPI NOR devices do not require a specific compatible, their ID can
in general be discovered with the JEDEC READ ID opcode. In this case,
only the "jedec,spi-nor" generic compatible is expected. Clarify this
information in the compatible description to (i) help device-tree
writers and (ii) prevent further attempts to extend this list with
useless information.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230616140054.2788684-1-miquel.raynal@bootlin.com


[ta: s/JEDEC/JEDEC SFDP for clarity and s/JEDEC READ ID/READ ID as
the opcode is not part of the JEDEC SFDP standard.]
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>