powerpc/powernv/ioda: Protect PE list

powerpc/powernv/ioda: Protect PE list

BugLink: https://bugs.launchpad.net/bugs/1862121
Protect the PHB's list of PE. Probably not needed as long as it was
populated during PHB creation, but it feels right and will become
required once we can add/remove opencapi devices on hotplug.

Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191121134918.7155-3-fbarrat@linux.ibm.com
(cherry picked from commit 80f1ff83fa113aa718ff4009684981386c574954)
Signed-off-by: Frank Heimes <frank.heimes@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>