openrisc: use new common dtc rule

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

This requires renaming arch/openrisc/boot/Makefile to
arch/openrisc/boot/dts/Makefile, and updating arch/openrisc/Makefile to
call the new Makefile.

Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>