MAINTAINERS: MediaTek: add USB related files

Add dt-binding files of xhci-mtk and phys, C files of
phys.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>