Enable usb low and full speed devices connected to the imx6 root hubs.

This enables the PHY circuitry for UTMI+ level 2 and 3, and sets the
flag to tell the ehci code that the root hub has a transaction translator
in it. For imx6 we can use the standard ehci_get_port_speed_portsc()
function to find out what speed device is connected to the port.