tzc400: correct FAIL_CONTROL Privileged bit

When bit 20 of TZC400 Fail control register is set to 1, it means
Privileged access, the macros FAIL_CONTROL_PRIV_PRIV and
FAIL_CONTROL_PRIV_UNPRIV are then updated to reflect this.

Change-Id: I01e522fded5cf66c9827293ddcf543c79f9e509e
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/183494


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>