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

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