SPM: Assign valid PID to agent partitions

According to FF-M, partition ID should be a positive
value. The agent partitions should also follow that.
Also there can be usage cases that the psa_notify() is
used to trigger the agent partitions. That also requires
the PID to be a positive value.

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: Ic9b1ef7caabb67eb9fca464b6e766be1c6a68381
4 files changed