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
diff --git a/docs/design_documents/index.rst b/docs/design_documents/index.rst
index f1d8386..11c1c5a 100644
--- a/docs/design_documents/index.rst
+++ b/docs/design_documents/index.rst
@@ -7,10 +7,12 @@
 
    cmake_framework
    measured_boot_poc
+   measured_boot_dtpm_poc
    drtm_poc
    rse
    psci_osi_mode
    measured_boot
+   dtpm_drivers
 
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