iommu/arm-smmu: set MAX_MASTER_STREAMIDS to MAX_PHANDLE_ARGS

The DT parsing code that determines stream IDs uses
of_parse_phandle_with_args and thus MAX_MASTER_STREAMIDS
is always bound by MAX_PHANDLE_ARGS.

Signed-off-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>