powerpc/mm: Reduce hashtable size when using 64kB pages

At the moment we size the hashtable based on 4kB pages / 2, even on a
64kB kernel. This results in a hashtable that is much larger than it
needs to be.

Grab the real page size and size the hashtable based on that

Note: This only has effect on non hypervisor machines.

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