dts: nxp: Refactor imx/rt dts.fixup into arch/arm/soc

Move common SoC dts.fixup defines into arch/arm/soc/nxp_imx/rt/dts.fixup
so we remove duplication in the boards and only have board specific
defines in boards/arm/<FOO>/dts.fixup.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>