xen/arm32: Add an helper to invalidate all instruction caches

xen/arm32: Add an helper to invalidate all instruction caches

This is similar to commit fb9d877a9c0f3d4d15db8f6e0c5506ea641862c6
"xen/arm64: Add an helper to invalidate all instruction caches"
except it is on ARM32 side.

When we are flushing the cache we are most likely also want
to flush the branch predictor too. Hence we add this.

And we also need to follow this with dsb()/isb() which are
memory barriers().

Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>