MAINTAINERS: update MediaTek PHY/USB entry

Due to the phy/usb bindings are converted into YAML schema and
also renamed, update entries.
Meanwhile add drivers/usb/host/mtk-xhci* files.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20201225075258.33352-11-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>