Extend SYSTEM_OFF test case

Extend SYSTEM_OFF to involve more than just the lead CPU.

The typical way to use SYSTEM_OFF is to use calls to CPU_OFF on
all online cores except for the last one, which instead uses SYSTEM_OFF.

test_system_off_cpu_other_than_lead() case is added to turn on any
random CPU other then lead CPU and perform SYSTEM_OFF from the CPU which
was turned ON.

Signed-off-by: Deepika Bhavnani <deepika.bhavnani@arm.com>
Change-Id: Ice62d0e7ef0db63ccb030e8dc1a83d9bd55e70f2