spi: spi-mtk-nor: support standard spi properties

Use default supports_op() to support spi-[rt]x-bus-width properties.
And check dummy op's byte length instead of its bus width for output.

Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
Link: https://lore.kernel.org/r/20200826091852.519138-1-ikjn@chromium.org


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