Revert "spd: trusty: only process one function ID at a time"

This reverts commit 64c07d0f009cdae746b227ae30055088bca93b50.
We should let Trusty do its own synchronization between
multiple guests. There is some already: Trusty can only execute
one std call at a time (regardless of who sent it), but it
can process multiple fastcalls.

Bug: 269285339
Change-Id: I42b7b69417b756fea9740ceb22ee462955ef2650
1 file changed