arch-power: Add fixed-point byte-reversed load and store instructions

arch-power: Add fixed-point byte-reversed load and store instructions

This adds the following load and store instructions:
  * Load Halfword Byte-Reverse Indexed (lhbrx)
  * Load Word Byte-Reverse Indexed (lwbrx)
  * Load Doubleword Byte-Reverse Indexed (ldbrx)
  * Store Halfword Byte-Reverse Indexed (sthbrx)
  * Store Word Byte-Reverse Indexed (stwbrx)
  * Store Doubleword Byte-Reverse Indexed (stdbrx)

Change-Id: I9f211bb4e3007ca09002a9ba4e5afb4b2e67cddd
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>