usb: host: ehci-vf: Implement board_usb_phy_mode weak function

Add board_usb_phy_mode weak function on similar lines to ehci-mx6.
However since Vybrid USB does not have a true OTG, make this weak
functon just return 0. The function is supposed to be implemented
by the individual boards using a GPIO for providing the OTG pin
functionality.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>