USB: add support for multiple PCI OHCI controllers

Add new configuration variable CONFIG_PCI_OHCI_DEVNO.
In case of several PCI USB controllers on a board this variable
specifys which controller to use.

Also add USB support for sokrates board.

See doc/README.generic_usb_ohci for details.

Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>
Signed-off-by: Markus Klotzbuecher <mk@denx.de>