arch-power: Introduce proper opcode fields

arch-power: Introduce proper opcode fields

This introduces separate extended opcode fields for DS, X, XFL,
XFX, XL and XO form instructions and renames the primary opcode
field from OPCODE to PO as listed in the Power ISA manual.

Scenarios where multiple instructions of different forms share
the same primary opcode have also been addressed by using the
correct extended opcode fields for decoding.

Change-Id: I4a01820f6a6326ef79330221b717952c6b9cbba3
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>