Add PSA FWU M update agent documentation

Document how the update agent uses the PSA Certified Firmware Update API
1.0 to implement Platform Security Firmware Update for the A-profile Arm
Architecture API.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I55a7b80727b4b63acdf7ed27d2ab956d4ad81dc3
9 files changed