target-ppc: Add xsmaxcdp and xsmincdp instructions

xsmaxcdp: VSX Scalar Maximum Type-C Double-Precision
xsmincdp: VSX Scalar Minimum Type-C 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>