hw/arm/smmuv3: Queue helpers

We introduce helpers to read/write into the command and event
circular queues.

smmuv3_write_eventq and smmuv3_cmq_consume will become static
in subsequent patches.

Invalidation commands are not yet dealt with. We do not cache
data that need to be invalidated. This will change with vhost
integration.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Prem Mallappa <prem.mallappa@broadcom.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1524665762-31355-7-git-send-email-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>