mx5: Add USB to Freescale MX51 defconfig

Update mx51_defconfig to include USB EHCI by default.

This patch applies to 2.6.34-rc6.

Signed-off-by: Dinh Nguyen <Dinh.Nguyen@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>