arch-arm: Secure EL2 checking

arch-arm: Secure EL2 checking

This patch adds Armv8.4-SecEL2 checking. Helpers implementing
EL2Enabled, IsSecureEL2Enabled and HaveSecureEL2Ext following
the architecture pseudocode are provided. These are intended
to be used for checking register access permissions.

Change-Id: I3d06d0127cf165c1eeaf3302830742d610cef719
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23766
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>