docs(fconf): update bindings for multi-RoT CoTs

Update CoT binding documentation to add the signing-key property
as optional in root-certificates and add rot_keys node

Change-Id: I1d1fbc0394275520cfa43213d5b7006e51990fdd
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
1 file changed