dt-bindings: spi: mxic: Convert to yaml

Straightforward conversion from regular text to yaml schema of the
Macronix SPI controller DT bindings.

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/20211216111654.238086-9-miquel.raynal@bootlin.com