usb: mtu3: add debugfs interface files

This adds more debugfs consumers. The debugfs entries read some
important registers, fifo status, QMU ring, endpoint status, and
IPPC probe interface to get internal status.
With these entries, users can check the registers, endpoint and
GPD used during run time.

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