phy: mediatek: add MediaTek T-PHY support for PCIe

The driver provides PHY for USB2, USB3.0, PCIe and SATA, and now
we just enable PCIe. As for the other functionalities will be
added gradually in upcoming days.

This is adapted from the Linux version.

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