MIPS: BCM63XX: Create platform_device for USBD

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Reviewed-by: Jonas Gorski <jonas.gorski@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4111/
Signed-off-by: John Crispin <blogic@openwrt.org>