dts: x86/atom: Refactor SoC dts.fixup into arch/x86/soc/atom

Move common SoC dts.fixup defines into arch/x86/soc/atom/dts.fixup so we
remove duplication in the boards and only have board specific defines in
boards/x86/<FOO>/dts.fixup.

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