Fix #6294: Full HW Random Generator

Change-Id: I9babada92991c646d844a25af175150d530a8ddb
Reviewed-on: https://gerrit.st.com/11597
Reviewed-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Tested-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Reviewed-by: Jocelyn RICARD <jocelyn.ricard@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>

Conflicts:
	core/arch/st231/plat-stm/rng_support.c
Fix #6323: A failing RSA decode leads to panic

Change-Id: Ia4762f076922338f280d431f104b653e731bf64f
Reviewed-on: https://gerrit.st.com/11598
Reviewed-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Tested-by: Jean-Michel DELORME <jean-michel.delorme@st.com>
Tested-by: Jocelyn RICARD <jocelyn.ricard@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>

Update wrt comments from review

Fix returned error when the mode is not ok