dm: ehci-mx6: support driver model

dm: ehci-mx6: support driver model

Support driver model for ehci mx6 driver.
Consolidate code to be shared between DM and non-DM, such as
introducing ehci_mx6_common_init.
For simplicity, some old fasion code are keeped for DM usage,
such as board_ehci_power and board_usb_phy_mode. And 'dr-mode',
usbphy and vbus handling code for DM is not added now.
These will be added in future patches.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Simon Glass <sjg@chromium.org>