SPM: Introduce new agent API

Initially, just wrap the existing SPM PSA API.
This mostly just helps to identify the callers and to nail down the
types needed.
Some fairly minor changes to the design documentation to match this
implementation.

No behavioural change.

Signed-off-by: Chris Brand <chris.brand@cypress.com>
Change-Id: Id2f36d10156e8ace55f9f50355e6e8ebe1d762b8
9 files changed