c6x: use new common dtc rule

The current rules have the .dtb files build in a different directory
from the .dts files. This patch changes c6x to use the generic dtb
rule which builds .dtb files in the same directory as the source .dts.

This requires moving parts of arch/c6x/boot/Makefile into newly created
arch/c6x/boot/dts/Makefile, and updating arch/c6x/Makefile to call the
new Makefile. linked_dtb.S is also moved into boot/dts/ since it's used
by rules that were moved.

Acked-by: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: linux-c6x-dev@linux-c6x.org
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>