usb:ehci-mx6 add phy mode query function

usb:ehci-mx6 add phy mode query function

usb_phy_enable should return status bit, but not phy mode bit, thus
add a new function usb_phy_mode to query the PHY for it's mode and
make usb_phy_enable just return 0 but not 'phy_ctrl & USBPHY_CTRL_OTG_ID'.

Include a new board weak function board_usb_phy_mode. If board code
does not reimplement this function, it just call usb_phy_mode and return
usb_phy_mode's return value. The reason to include such a weak function
is: " SOC OTG core <--connect--> board HOST port, but no pin id for
the board host port, so board can not use usb_phy_mode to return the
phy mode, but define it's own rule."

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Signed-off-by: Ye Li <B37916@freescale.com>