trusty: Always call back into trusty on interrupted CPUs

Trigger a trusty nop every time a std-call is interrupted to make sure
the interrupted cpu will eventually re-enter trusty even if the std-call
resumes on another cpu.

Bug: 145619829

Change-Id: I498dcea056a675d8374fe0d81908c61a1390e257
Signed-off-by: Arve Hjønnevåg <arve@android.com>