refact(trng): cleanup TRNG service tests

This patch adds the following changes:
1. Updates tftf-docs on the TRNG test support
2. Improvise few other files and macro names

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I69bbcb24e8485d2cd1384970e7424541d11838af
4 files changed