usb: ehci-mx6: handle vbus-supply

Drop board_ehci_power when dm usb used and switch to use
regulator api to handle vbus.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>