drivers: usb_dc_mcux: Enable second USB EHCI

Enable second usb EHCI if usb2 node has status="okay" Note that this
driver is still an single instance driver, this change simply enables
the driver to work with the usb2 peripheral if that one is enabled, and
usb1 is disabled.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>