dt-bindings: phy: Add document for phy-mtk-ufs

Add UFS M-PHY node document for MediaTek SoC chips.

Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>