phy: phy-mtk-tphy: add set_mode callback

This is used to force PHY with USB OTG function to enter a specific
mode, and override OTG IDPIN(or IDDIG) signal.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>