usb: ehci-mx6: Use portnr from DT in DM case

In case the platform uses DM, determine port number, which is
used as offset in USBMISC registers, from PHY node DT aliases,
just like Linux does.

Fixes: 4de51cc2 ("usb: ehci-mx6: Drop assignment of sequence number")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>