Even though a SMMUv3 PMCG implementation may use an MSI as the form of
interrupt source, the kernel would still complain that it does not find
the wired (GSIV) interrupt in this case:

root@(none)$ dmesg | grep arm-smmu-v3-pmcg | grep "not found"
[   59.237219] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.8.auto: IRQ index 0 not found
[   59.322841] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.9.auto: IRQ index 0 not found
[   59.422155] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.10.auto: IRQ index 0 not found
[   59.539014] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.11.auto: IRQ index 0 not found
[   59.640329] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.12.auto: IRQ index 0 not found
[   59.743112] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.13.auto: IRQ index 0 not found
[   59.880577] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.14.auto: IRQ index 0 not found
[   60.017528] arm-smmu-v3-pmcg arm-smmu-v3-pmcg.15.auto: IRQ index 0 not found

Use platform_get_irq_optional() to silence the warning.

If neither interrupt source is found, then the driver will still warn that
IRQ setup errored and the probe will fail.
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Will Deacon <will@kernel.org>