Add reference platform diagram

Extends the introduction documentation to include a diagram that
illustrates the OP-TEE based reference platform with PSA root-of-trust
and UEFI SMM services.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: Ibd557c1c6e4e92e9cabfb637a8acc8fb3a807c8d
2 files changed