EBBR.yaml: add one setstate warning as known sct limitation

A specific case where SetState() return "unsupported" makes the SCT emit a
warning. As this is a valid return value, add a rule to force this warning
as "known sct limitation".

While at it, document this new "known sct limitation" category in the
README.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>