feat: add build flag to control support for CPU_OFF psci msg

Please check the inline comments for detailed description of the
new build flag. This flag will help us to create a negative test
scenario where we can exercise sending DENIED response to SPMC,
ultimately leading to a panic by SPMD.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I30d2ea7f3fbf09e4e4febceb85f045e25f3e6035
5 files changed