dt-bindings: interrupt-controller: arm,gic-v3: Make 'interrupts' optional

A GICv3 implementation without virtualization, such as the base QEMU
virt machine (without -M virtualization=on), does not issue maintenance
interrupts. Therefore its device-tree node does not need an 'interrupts'
property. Currently, validating the QEMU virt device-tree throws a
warning that 'interrupts' is missing. Make it optional.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220822152224.507497-3-jean-philippe@linaro.org