arch-power: Rename program counter registers

arch-power: Rename program counter registers

The Power ISA specification lists the Program Counter (PC) and
the Next Program Counter (NPC) registers as Current Instruction
Address (CIA) and Next Instruction Address (NIA). This applies
the ISA naming convention for these two registers.

Change-Id: I8b9094ab1c809f4dfdb4d7330c17f360adf063e9
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/16603
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>