hw/arm/smmu: Introduce smmu_get_iotlb_key()

Introduce the smmu_get_iotlb_key() helper and the
SMMU_IOTLB_ASID() macro. Also move smmu_get_iotlb_key and
smmu_iotlb_key_hash in the IOTLB related code section.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200728150815.11446-4-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>