ARM: mxs: add initial device tree support for imx28-evk board

This patch includes basic dt support which can boot via nfs rootfs.

Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>