refactor(trng): cleanup the existing TRNG support

This patch adds the following changes to complete the existing
TRNG implementation:

1. Adds a feature specific scope for buildlog generation.
2. Updates the docs on the build flag "TRNG_SUPPORT" and its values.
3. Makefile update and improves the existing comments at few sections
for better understanding of the underlying logic.

Change-Id: I3f72f0ccd5c94005a2df87158cf23199d2160d37
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/changelog.yaml b/changelog.yaml
index 8fcb217..83be618 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -595,6 +595,9 @@
       - title: DRTM
         scope: drtm
 
+      - title: TRNG
+        scope: trng
+
   - title: Libraries
 
     subsections: