CONFIG_USB_EHCI_MXC selects the old i.mx USB driver, which does not support
device tree.

Select the USB chipidea driver instead, so that USB can be functional on i.mx.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Olof Johansson <olof@lixom.net>