spi: Add compatible for Mediatek MT8186

This commit adds dt-binding documentation of spi bus for Mediatek MT8186 SoC
Platform.

Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220125012330.13449-4-leilk.liu@mediatek.com


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