feat(docs): update mboot threat model with dTPM

Add the discrete TPM to the TCG event log section of the measured boot
threat model. Include the example of a physical vurnerability that can
be used to compromise a dTPM.

Signed-off-by: Abhi Singh <abhi.singh@arm.com>
Change-Id: I2c06edf5e9031adc970c24426a8ae52b06efb614
1 file changed