ARM: mxs: rename dts files with soc name being the prefix

Rename mxs dts files with soc name being the prefix, so that the board
dts file can be located easily by soc name, and we also gain the
consistency of naming.

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>