dt-bindings: spi: mxic: Document the nand-ecc-engine property

This SPI controller supports interacting with an external ECC
engine. The nand-ecc-engine property already exist in the NAND world but
also applies to SPI controller nodes which have external correction
capabilities like Macronix's.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-10-miquel.raynal@bootlin.com