arch-arm: isSecureBelow from armarm pseudocode

arch-arm: isSecureBelow from armarm pseudocode

This patch introduces the inSecureBelow pseudocode function
defined in the armarm documentation. It also replaces the
inSecureState function call which was improperly used in
ELIs32: we might be in secure state (EL3), but with non-secure
lower ELs (SCR.NS = 1).

Change-Id: I01febcb54392ad4e51e785b4d5153aeb3437c778
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Chuan Zhu <chuan.zhu@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7221
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>