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

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 <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Signed-off-by: David Gibson <[email protected]>
(cherry picked from commit 230bf719d3a3b144a4ffa441e5d6170ef0ad8999)