xtensa: rename prom_update_property to of_update_property

This rename happened in 79d1c71 powerpc+of: Rename the drivers/of prom_*
functions to of_*.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
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.

Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Stephen Warren <swarren@nvidia.com>