spi: Convert spi-slave-mt27xx to json-schema

Convert Mediatek ARM SOC's SPI Slave controller binding
to json-schema format.

Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220125012330.13449-2-leilk.liu@mediatek.com


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