arch-power: Add fixed-point doubleword load and store instructions

arch-power: Add fixed-point doubleword load and store instructions

This adds the following load and store instructions:
  * Load Doubleword (ld)
  * Load Doubleword Indexed (ldx)
  * Load Doubleword with Update (ldu)
  * Load Doubleword with Update Indexed (ldux)
  * Store Doubleword (std)
  * Store Doubleword Indexed (stdx)
  * Store Doubleword with Update (stdu)
  * Store Doubleword with Update Indexed (stdux)

Change-Id: I57a95003b6c6cfc09cc40f9ac03b32a8dfd7b26d
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>