dt-bindings: mtd: convert nxp-spifi.txt to yaml format

Convert nxp-spifi.txt to yaml format.

Additional changes:
- ref /schemas/spi/spi-controller.yaml.
- remove label in example.
- change node name to spi in example.
- remove child node in example.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>