usb: ehci-mx6: Split ehci_mx6_common_init()

usb: ehci-mx6: Split ehci_mx6_common_init()

In order to pass component addresses around easily instead of passing
ad-hoc sequence numbers, it is necessary to split ehci_mx6_common_init().
Make it so and call the separate functions instead.

Since board_ehci_hcd_init() makes no sense in DM case, do not call it
in DM case.

Fixes: de8bfe098ed ("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>