arch-power: Add doubleword shift instructions

arch-power: Add doubleword shift instructions

This introduces a new class and a new format for XS form
instructions where the shift amount is specified by two
fields that must be concatenated and adds the following
instructions.
  * Shift Left Doubleword (sld[.])
  * Shift Right Doubleword (srd[.])
  * Shift Right Algebraic Doubleword (srad[.])
  * Shift Right Algebraic Doubleword Immediate (sradi[.])
  * Extend-Sign Word and Shift Left Immediate (extswsli[.])

Change-Id: If51c676009ddafb40f855b66c00eeeffa5d8874c
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>