dts: nxp: Refactor kwx dts.fixup into arch/arm/soc

Move common SoC dts.fixup defines into
arch/arm/soc/nxp_kinetis/kwx/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>