dt-bindings: spi: Convert spi-mtk-nor to json-schema

Convert Mediatek ARM SOC's serial NOR flash controller binding
to json-schema format.

Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200826085450.508556-1-ikjn@chromium.org


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