arch-arm: reg access permissions highest EL helper

arch-arm: reg access permissions highest EL helper

This patch implements a helper function to filter a register access
permissions by the highest EL implemented by the system.
This filtering is convenient to follow the architecture pseudocode.

Change-Id: Iedfb2d8624c926f2f0a9326f8b1b073ea9424ab9
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24663
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>