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>

include/configs/MPC8641HPCN.h

@@ -346,6 +346,7 @@ extern unsigned long get_board_sys_clk(unsigned long dummy);
 #define CFG_USB_EVENT_POLL	1
 #define CFG_USB_OHCI_SLOT_NAME 	"ohci_pci"
 #define CFG_USB_OHCI_MAX_ROOT_PORTS 15
+#define CFG_OHCI_SWAP_REG_ACCESS	1
 
 #if !defined(CONFIG_PCI_PNP)
     #define PCI_ENET0_IOADDR	0xe0000000
@@ -544,6 +545,7 @@ extern unsigned long get_board_sys_clk(unsigned long dummy);
     #define CONFIG_CMD_PCI
     #define CONFIG_CMD_SCSI
     #define CONFIG_CMD_EXT2
+    #define CONFIG_CMD_USB
 #endif