arm: mvebu: dts: Build only arch-compatible dts files

64-bit Armada DTS files are not build correctly during compilation of
32-bit Armada boards and vice versa. So fix makefile build system to
compile only those dts files which are compatible for the current build
(64-bit Armada DTS files only for 64-bit builds and 32-bit Armada DTS files
only for 32-bit builds).

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>