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/measured_boot.rst b/docs/design_documents/measured_boot.rst
index 005903e..1f76770 100644
--- a/docs/design_documents/measured_boot.rst
+++ b/docs/design_documents/measured_boot.rst
@@ -91,6 +91,14 @@
    and the variable length crypto agile structure called TCG_PCR_EVENT2. Event
    Log driver implemented in TF-A covers later part.
 
+#. Discrete TPM
+
+   A Discrete TPM (Trusted Platform Module) can be used alongside Event Log to
+   extend measurements and validate Measured Boot functionality. The use of a
+   Discrete TPM in TF-A to extend measurements of images and other critical data
+   allows for an additional layer of security. The TPM can be used to attest the
+   integrity of the Event Log.
+
 #. |RSE|
 
    It is one of the physical backends to extend the measurements. Please refer
@@ -229,7 +237,7 @@
 
 --------------
 
-*Copyright (c) 2023, Arm Limited. All rights reserved.*
+*Copyright (c) 2023-2025, Arm Limited. All rights reserved.*
 
 .. _Arm® Server Base Security Guide: https://developer.arm.com/documentation/den0086/latest
 .. _TCG EFI Protocol Specification: https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf