Fix USB support issue for MPC8641HPCN board.

The configuration file has already enabled USB, but it
missed definition of CFG_OHCI_SWAP_REG_ACCESS, the USB
on MPC8641HPCN can not work because of the wrong USB
register endian.

And add the USB command to U-Boot commands list.

Signed-off-by: Zhang Wei <wei.zhang@freescale.com>