PCI: rcar: Convert to MSI domains

In anticipation of the removal of the msi_controller structure, convert
the Rcar host controller driver to MSI domains.

We end-up with the usual two domain structure, the top one being a
generic PCI/MSI domain, the bottom one being Rcar-specific and handling
the actual HW interrupt allocation.

Link: https://lore.kernel.org/r/20210330151145.997953-4-maz@kernel.org
Tested-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
[lorenzo.pieralisi@arm.com: merged fix https://lore.kernel.org/linux-pci/87y2e2p9wk.wl-maz@kernel.org]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>

Author: Marc Zyngier

drivers/pci/controller/Kconfig

@@ -58,7 +58,6 @@ config PCIE_RCAR_HOST
 	bool "Renesas R-Car PCIe host controller"
 	depends on ARCH_RENESAS || COMPILE_TEST
 	depends on PCI_MSI_IRQ_DOMAIN
-	select PCI_MSI_ARCH_FALLBACKS
 	help
 	  Say Y here if you want PCIe controller support on R-Car SoCs in host
 	  mode.