spi: dt-bindings: add binding doc for spi-mtk-snfi

Add device-tree binding documentation for Mediatek SPI-NAND Flash
Interface.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220424032527.673605-5-gch981213@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>