of: merge of_find_node_by_phandle

Merge common function between powerpc, sparc and microblaze. Code is
identical for powerpc and microblaze, but adds a lock (and release) of
the devtree_lock on sparc.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>