powerpc/numa: Look for ibm, associativity-reference-points at the root

If we don't find ibm,associativity-reference-points as a child of
/rtas, look for it at the root of the tree instead. We use this on
Book3E where we have no RTAS but still use the sPAPR conventions
for NUMA.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>