MIPS: ath79: add common USB Host Controller device

Add common platform_device and helper code to make the registration of
the built-in USB controllers easier on the board which are using them.
Also register the USB controller on the AP81 and PB44 boards.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2442/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>