powerpc/powernv/ioda: Protect PE list

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