arch-power: Refactor load-store instructions

arch-power: Refactor load-store instructions

This changes the base classes for load-store instructions
and introduces two new classes for DS form instructions
which use a shifted signed immediate field as the offset
from the base address and for X form instructions which
use registers for both the offset and the base address.
The formats have also been updated to make use of the new
base classes.

Change-Id: Ib5d1bb5d7747813e0e5b1e3075489f1a3aa72660
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>