docs(threat-model): make measured boot out of scope

Add an explicit note that measured boot is out of scope of the threat
model. For example, we have no threat related to the secure
management of measurements, nor do we list its security benefits
(e.g. in terms of repudiation).

This might be a future improvement to the threat model but for now
just acknowledge it is not considered.

Change-Id: I2fb799a2ef0951aa681a755a948bd2b67415d156
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>