global: Clean up arch/*/dts/Makefile

With commit 3609e1dc ("dts: automatically build necessary .dtb files")
we now have logic that will ensure that all device trees needed in the
binary are built automatically. Any device tree that the developer needs
while working can still be built normally via make arch/.../foo.dtb so
we can simply drop the rest of this logic.

Suggested-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Tom Rini <trini@konsulko.com>