PCI: rockchip-dwc: Add legacy interrupt support

The legacy interrupts on the rk356x PCIe controller are handled by a
single muxed interrupt. Add IRQ domain support to the pcie-dw-rockchip
driver to support the virtual domain.

Link: https://lore.kernel.org/r/20220429123832.2376381-4-pgwipeout@gmail.com


Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>