usb: dwc2: add T: line to MAINTAINERS showing Felipe's tree

Starting with v3.18-rc, patches for dwc2 will go through Felipe's
tree. Add a T: line to MAINTAINERS to document this.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>