dt-bindings: phy: add a document for MediaTek tphy

This adds a document for tphy which supports physical layer
functionality for a number of controllers on MediaTek SoCs,
such as, USB2.0, USB3.0, PCIe, and SATA.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>