at91: add CONFIG_USB_OHCI_HCD_AT91 to usb-ohci

The at91 OHCI driver has a separate Kconfig symbol with newer kernels,
include it.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42411 3c298f89-4303-0410-b956-a3cf2f4a3e73