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/global_substitutions.txt b/docs/global_substitutions.txt
index 23a91cd..ecf6d63 100644
--- a/docs/global_substitutions.txt
+++ b/docs/global_substitutions.txt
@@ -6,6 +6,7 @@
 .. |BTI| replace:: :term:`BTI`
 .. |CoT| replace:: :term:`CoT`
 .. |COT| replace:: :term:`COT`
+.. |CRB| replace:: :term:`CRB`
 .. |CSS| replace:: :term:`CSS`
 .. |CVE| replace:: :term:`CVE`
 .. |DICE| replace:: :term:`DICE`
@@ -19,11 +20,13 @@
 .. |FCONF| replace:: :term:`FCONF`
 .. |FDT| replace:: :term:`FDT`
 .. |FF-A| replace:: :term:`FF-A`
+.. |FIFO| replace:: :term:`FIFO`
 .. |FIP| replace:: :term:`FIP`
 .. |FVP| replace:: :term:`FVP`
 .. |FWU| replace:: :term:`FWU`
 .. |GIC| replace:: :term:`GIC`
 .. |HES| replace:: :term:`HES`
+.. |I2C| replace:: :term:`I2C`
 .. |ISA| replace:: :term:`ISA`
 .. |Linaro| replace:: :term:`Linaro`
 .. |MMU| replace:: :term:`MMU`
@@ -55,6 +58,7 @@
 .. |SP| replace:: :term:`SP`
 .. |SPD| replace:: :term:`SPD`
 .. |SPM| replace:: :term:`SPM`
+.. |SPI| replace:: :term:`SPI`
 .. |SRTM| replace:: :term:`SRTM`
 .. |SSBS| replace:: :term:`SSBS`
 .. |SVE| replace:: :term:`SVE`