spi: mediatek: Add bindings for mediatek MT2712 soc platform

This patch adds a DT binding documentation for the MT2712 soc.

Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>