target/ppc/spapr_caps: Add new tristate cap safe_indirect_branch

Add new tristate cap cap-ibs to represent the indirect branch
serialisation capability.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>