powerpc/rtas_flash: New return code to indicate FW entitlement expiry

Add new return code to rtas_flash to indicate firmware entitlement
expiry. Strictly we don't need this update. But to keep it in sync
with PAPR, this was added.

Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>