xen/arm: smmuv1: Fixed stream matching register allocation

xen/arm: smmuv1: Fixed stream matching register allocation

SMR allocation should be based on the number of supported stream
matching register for each SMMU device.

Issue introduced by commit 5e08586afbb90b2e2d56c175c07db77a4afa873c
when backported the patches from Linux to XEN to fix the stream match
conflict issue when two devices have the same stream-id.

Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Rahul Singh <rahul.singh@arm.com>