hw/arm/smmuv3: Remove SMMUNotifierNode

The SMMUNotifierNode struct is not necessary and brings extra
complexity so let's remove it. We now directly track the SMMUDevices
which have registered IOMMU MR notifiers.

This is inspired from the same transformation on intel-iommu
done in commit b4a4ba0d
("intel-iommu: remove IntelIOMMUNotifierNode")

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-id: 20190409160219.19026-1-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>