arch-arm: Trap to EL2 only if not in Secure State

arch-arm: Trap to EL2 only if not in Secure State

MRS/MSR Instructions should trap to EL2 only if we are in non-Secure
state since at the current implementation (Armv8.0) there is no Secure
EL2.

Change-Id: I93af415fbcbd19a470752adf6afc92e520e9645d
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13777
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>