hw/ppc/spapr: Move code related to "ibm,pa-features" to a separate function

The function spapr_populate_cpu_dt() has become quite big
already, and since we likely have to extend the pa-features
property for every new processor generation, it is nicer
if we put the related code into a separate function.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 230bf719)