MLK-25483 usb: ehci-mx6: Enable USB PHY clock for iMX8DXL

MLK-25483 usb: ehci-mx6: Enable USB PHY clock for iMX8DXL

On iMX8DXL, two USB controllers share clock (enabled by SCFW) for
controller but have dedicated LPCG clock for PHY.
The PHY LPCG will be cleared once the PHY is power down. So without
explictly enabling PHY LPCG clock, USB PHY access will cause
u-boot crash.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>