ARM: dts: imx: add device tree support for Freescale imx50evk board

Add device tree support for the Freescale IMX50EVk board based around the
IMX50 SoC. Supports UART, SPI flash, FEC ethernet and USB on this board.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>