target-ppc: implement lxsd and lxssp instructions

target-ppc: implement lxsd and lxssp instructions

lxsd: Load VSX Scalar Dword
lxssp: Load VSX Scalar Single

Moreover, DS-Form instructions shares the same primary opcode, bits
30:31 are used to decode the instruction. Use a common routine to decode
primary opcode(0x39) - ds-form instructions and branch-out depending on
bits 30:31.

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