arch-power: Add doubleword load-store instructions

arch-power: Add doubleword load-store instructions

This introduces new formats for DS form instructions and
adds the following 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: I2a88364e82a11685e081f57be5fd5afd44335668
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>