MAINTAINERS: Add file patterns for usb device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -11956,6 +11956,7 @@ L:	linux-usb@vger.kernel.org
 W:	http://www.linux-usb.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
 S:	Supported
+F:	Documentation/devicetree/bindings/usb/
 F:	Documentation/usb/
 F:	drivers/usb/
 F:	include/linux/usb.h