arch-arm: Adding MiscReg Priv (EL1) global flag

arch-arm: Adding MiscReg Priv (EL1) global flag

This patch introduces a single global flag for setting RW access
permission at EL1 level, in either secure and non-secure mode.

Change-Id: I35df66a73349044ca996b5c04c5a2476f3a7abdf
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10042
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>