of: remove unused extern reference to devtree_lock

Neither the powerpc nor the microblaze code use devtree_lock anymore.
Remove the extern reference.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michal Simek <monstr@monstr.eu>