ppc/pnv: generate an OEM SEL event on shutdown

OpenPOWER systems expect to be notified with such an event before a
shutdown or a reboot. An OEM SEL message is sent with specific
identifiers and a user data containing the request : OFF or REBOOT.

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