docs(tpm): add design documentation for dTPM

-documentation for Discrete TPM drivers.
-documentation for a proof of concept on rpi3;
 Measured Boot using Discrete TPM.

Signed-off-by: Abhi Singh <abhi.singh@arm.com>
Change-Id: If8e7c14a1c0b9776af872104aceeff21a13bd821
8 files changed