of: put default string compare and #a/s-cell values into common header

Most architectures don't need to change these.  Put them into common
code to eliminate some duplication

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