Add Secure Partition Manager (SPM) design document

This patch adds documentation that describes the design of the Secure
Partition Manager and the specific choices in their current
implementation.

The document "SPM User Guide" has been integrated into the design
document.

Change-Id: I0a4f21a2af631c8aa6c739d97a5b634f3cb39991
Co-authored-by: Achin Gupta <achin.gupta@arm.com>
Co-authored-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>