mx5: Remove CFG_MXC_USB_PORT

CFG_MXC_USB_PORT is not used anywhere, so remove this unused symbol.

Suggested-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>