spi: dt-bindings: mediatek,spi-mtk-nor: Update bindings for nor flash

The spi-mtk-nor controller of mt8173, mt8186 and mt8192 have their
DT data. They don't use mt8173 as fallback.

Using the fallback of mt8186 to enables the controllers to support
mt8188.

Not all of spi-mtk-nor controller need interrupt property, so we
don't mark interrupt as required.

Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Link: https://lore.kernel.org/r/20220715115443.4154-1-allen-kh.cheng@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>