[TF-A] sp select in atf

Hello all, I see the atf use different sp, and special for smc64 it use the sp_el0. So for the unhandled-exception handler, it must switch to sp_el0 to do the back-trace-dump. Since it default used sp_el3 when exception happen.

My question here is why using different sp in atf code? Just use the sp_el3 for all scenes seems more simpler.

Cheers, - feng