ppc4xx: Use generic usb-ohci driver for sequoia board

This patch makes the sequoia board use the generic usb-ohci driver
instead of cpu/ppc4xx/usb_ohci.c.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Signed-off-by: Stefan Roese <sr@denx.de>