target-ppc: introduce opc4 for Expanded Opcode

ISA 3.0 has introduced EO - Expanded Opcode. Introduce third level
indirect opcode table and corresponding parsing routines.

EO (11:12) Expanded opcode field
Formats: XX1

EO (11:15) Expanded opcode field
Formats: VX, X, XX2

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
[dwg: Trivial checkpatch fixup]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
