arch-power: Fix load-store instructions

arch-power: Fix load-store instructions

Now that 64-bit registers and operands are being used, the
instructions for words must read or write just one word at
a time. This fixes the following instructions.
  * Load Word and Zero (lwz)
  * Load Word and Zero Indexed (lwzx)
  * Load Word and Zero with Update (lwzu)
  * Load Word and Zero with Update Indexed (lwzux)
  * Load Word And Reserve Indexed (lwarx)
  * Store Word (stw)
  * Store Word Indexed (stwx)
  * Store Word with Update (stwu)
  * Store Word with Update Indexed (stwux)
  * Store Word Conditional Indexed (stwcx.)

This also fixes decoding of load-store update instructions
for some special scenarios when RA is zero or RA and RT
are the same. In such cases, the instruction is considered
invalid.

Change-Id: I6787d3614ba8f1b1cbf30a49f85ef422324d7c21
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>