xen/arm: Don't boot Xen on platform using AIVIVT instruction caches

xen/arm: Don't boot Xen on platform using AIVIVT instruction caches

The AIVIVT is a type of instruction cache available on Armv7. This is
the only cache not implementing the IVIPT extension and therefore
requiring specific care.

To simplify maintenance requirements, Xen will not boot on platform
using AIVIVT cache.

This should not be an issue because Xen Arm32 can only boot on a small
number of processors (see arch/arm/arm32/proc-v7.S). All of them are
not using AIVIVT cache.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>