Since the i.MX8MM reuses the otg controllers on i.MX7D. We can use
CONFIG_USB_EHCI_MX7 for them.

Due the TCPC and load switch are used on Typec circuit. Add the
board_usb_init and board_usb_cleanup to ehci-mx6 DM driver. So
we can implement the TCPC settings in these board functions.

Signed-off-by: Ye Li <ye.li@nxp.com>