arch-arm: Implement AArch32 RVBAR

arch-arm: Implement AArch32 RVBAR

RVBAR has been added to the system register list since ARMv8.0-A.  It is
implemented only if the highest Exception Level is different (minor)
than EL3.  If that's not the case, MVBAR is used. Since the two
registers are mutually exclusive (depending on the presence of EL3),
they share the same coprocessor numbers:

p15, 0, c12, c0, 1

Rather than introducing a new register alias, we overload MVBAR so that
it is treated as RVBAR if ArmSystem::highestEL() < EL3. This patch is
changing the MiscReg info so that EL1 or EL2 access MVBAR (as RVBAR).

N.B MVBAR is RW, whereas RVBAR is RO

Change-Id: Ida3070413fd151ce79c446e99a2a389298d5f5bd
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/13999
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>