powerpc/xive: add OPAL extensions for the XIVE native exploitation support

The support for XIVE native exploitation mode in Linux/KVM needs a
couple more OPAL calls to get and set the state of the XIVE internal
structures being used by a sPAPR guest.

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