target/ppc: Move page size setup to helper function

target/ppc: Move page size setup to helper function

Initialization of the env->sps structure at the end of instance_init is
specific to the 64-bit hash MMU, so move the code into a helper function
in mmu-hash64.c.

We also create a corresponding function to be called at finalize time -
it's empty for now, but we'll need it shortly.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>