Add TRNG Firmware Interface service

This adds the TRNG Firmware Interface Service to the standard
service dispatcher. This includes a method for dispatching entropy
requests to platforms and includes an entropy pool implementation to
avoid dropping any entropy requested from the platform.

Change-Id: I71cadb3cb377a507652eca9e0d68714c973026e9
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/docs/global_substitutions.txt b/docs/global_substitutions.txt
index d33155b..24ac830 100644
--- a/docs/global_substitutions.txt
+++ b/docs/global_substitutions.txt
@@ -56,6 +56,7 @@
 .. |TF-M| replace:: :term:`TF-M`
 .. |TLB| replace:: :term:`TLB`
 .. |TLK| replace:: :term:`TLK`
+.. |TRNG| replace:: :term:`TRNG`
 .. |TSP| replace:: :term:`TSP`
 .. |TZC| replace:: :term:`TZC`
 .. |UBSAN| replace:: :term:`UBSAN`