arch-arm: Fix secure MiscReg access when EL3 is not AArch32

arch-arm: Fix secure MiscReg access when EL3 is not AArch32

When EL3 is not implemented or it is running on AArch64, Secure banking
does not apply and there is only one flatten register version.  In this
scenario gem5 is using the _NS (Non-secure) version as a default backing
storage location: secure mode software must be able to access the
non-secure register.

Change-Id: I5086e6228a5cba4d18c632543a2bcf80ffb069a8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9941
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>