microblaze: use new common dtc rule

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

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

Cc: Michal Simek <monstr@monstr.eu>
Cc: microblaze-uclinux@itee.uq.edu.au
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>