spapr: Make a helper to set up cpu entry point state

spapr: Make a helper to set up cpu entry point state

Under PAPR, only the boot CPU is active when the system starts.  Other cpus
must be explicitly activated using an RTAS call.  The entry state for the
boot and secondary cpus isn't identical, but it has some things in common.
We're going to add a bit more common setup later, too, so to simplify
make a helper which sets up the common entry state for both boot and
secondary cpu threads.

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