dt-bindings: iommu: arm,smmu-v3: make PRI IRQ optional

The Page Request Interface (PRI) is an optional PCIe feature. As such, a
SMMU would not need to handle it if the PCIe host bridge or the SMMU
itself do not implement it. Also an SMMU could be connected to a platform
device, without any PRI functionality whatsoever.
In all cases there would be no SMMU PRI queue interrupt to be wired up
to an interrupt controller.
At the moment, with the "eventq,gerror,priq,cmdq-sync" order, we
would need to sacrifice the command queue sync interrupt as well, which
might not be desired.

Relax the binding to allow specifying certain useful combinations of
wired interrupts, for instance just the "gerror" interrupt, or omitting
both "pri" and "cmdq-sync".

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-2-andre.przywara@arm.com