doc: secure partition manager design ...

Former EL3 Secure Partition Manager using MM protocol is renamed
Secure Partition Manager (MM).
A new Secure Partition Manager document covers TF-A support for the
PSA FF-A compliant implementation.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I9763359c2e96181e1726c8ad72738de293b80eb4