ARM: dts: imx: add IMX50 SoC device tree

Create device tree source for the Freescale IMX50 SoC. This was based on
the IMX53 source with changes made as necessary.

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