target-ppc: Add xsmaxjdp and xsminjdp instructions

xsmaxjdp: VSX Scalar Maximum Type-J Double-Precision
xsminjdp: VSX Scalar Minimum Type-J Double-Precision

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>