ar71xx: Chipidea USB device support

Changes the platform to use the Chipidea driver instead of the
generic USB host driver which has support for both host and
device modes (selected on boot).

The changes in 930-chipidea-pullup.patch are already in mainline.
I'll upstream 920-usb-chipidea-AR933x-platform-support.patch once I
can test the changes with a newer kernel.

Signed-off-by: Svetoslav Neykov <svetoslav@neykov.name>