xtensa: use new common dtc rule

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

This requires moving parts of arch/xtensa/boot/Makefile into newly
created arch/xtensa/boot/dts/Makefile, and updating arch/xtensa/Makefile
to call the new Makefile.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Chris Zankel <chris@zankel.net>