ARM: dts: imx: use nodes label in board dts

Following omap3-evm.dts way, it changes all imx dts files to use label
in board dts to refer to nodes defined by soc dtsi.  Thus, the board
dts files become easier to read and edit with the least indentation
levels.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>