powerpc/numa: Add length when creating OF properties via VPHN

The rest of the NUMA code expects an OF associativity property with
the first cell containing the length. Without this fix all topology changes
cause us to misparse the property and put the cpu into node 0.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>