MIPS: ath79: add AR933X specific USB platform device registration

Also select the USB_ARCH_HAS_EHCI symbol in order to make the
EHCI driver available.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Kathy Giori <kgiori@qca.qualcomm.com>
Cc: "Luis R.  Rodriguez" <rodrigue@qca.qualcomm.com>
Patchwork: https://patchwork.linux-mips.org/patch/2527/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>