dt-bindings: spi: update bindings for MT6765 SoC

Add a DT binding documentation for the MT6765 soc.

Signed-off-by: luhua.xu <luhua.xu@mediatek.com>
Link: https://lore.kernel.org/r/1568195731-3239-2-git-send-email-luhua.xu@mediatek.com


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