Update service deployment documentation

Updates the old portability model page to better describe how
service provider deployments for different environments and
platforms are organised. Introduces a more flexible directory
structure that improves reuse and extensibility for service
provider deployments.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I5635c38210cbb55d993150e8efe9e84b95acdf49
8 files changed