dts: arm: Refactor SoC dts.fixup into arch/arm/soc

Move common SoC dts.fixup defines into arch/arm/soc/<SOC>/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>