arch-power: Use 64-bit registers and operands

arch-power: Use 64-bit registers and operands

This increases the width of the general-purpose registers
and some of the special purpose registers to 64 bits in
accordance with the newer versions of the Power ISA and
enables usage in both 32-bit and 64-bit execution modes.
In 32-bit mode, the use of upper word is dependent on the
instruction being executed and in some cases this may be
undefined.

Change-Id: I2a5865a66e4ceab45e42a833d425abdd6bd6bf55
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>