usb: xhci-mtk: make IPPC register optional

Make IPPC register optional to support host side of dual-role mode,
due to it is moved into common glue layer for simplification.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>