trusty: Add smp support

Add an unlocked nop call to allow multiple cpus to enter trusty.
Other standard calls are still serialized to avoid return codes
getting mixed up.
A new return code is used to indicate that the standard call is
running on another cpu.

Change-Id: Iccc0785138fbc3eb9daebf22fd79d6c1f9b23f6a
Signed-off-by: Arve Hjønnevåg <arve@android.com>