ARM: dts: imx27: Place the usb phy nodes in the board dts files

It is not a good approach to have the USB PHY nodes inside imx27.dtsi since
the USB PHYs on mx27 are not internal to the SoC.

Place the USB PHY nodes in the board dts files instead.

Also, each board may have a different clock source for the USB PHY, so do not
hardcode it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>