arch-power: Added support for SMT

arch-power: Added support for SMT

This adds support for SMT (simultaneous multi-threading).
  * Enable multithreading for power architecture.
  * Make number of threads 2.
  * Added support for post and clear interrupt.
  * Added interrupt array support for external interrupts so
    that it will check any pending interrupt request in every
    tick.
  * Initialize PIR for second thread.
  * Initialize PC state for second thread so that it
    will loop until first thread try to wake up secondary
    threads.
  * Initialize register r3 for second thread.

Change-Id: I5825e62a94b50aaa213eb64335973129addf2e4c
Signed-off-by: Kajol Jain <kajoljain797@gmail.com>