PCI: rcar: Add gen2 device tree support for r8a7793

Add "renesas,pci-r8a7793" as a compatibility string for
"renesas,pci-rcar-gen2".

This doesn't change the driver, so it does nothing by itself. But it
does mean that checkpatch won't complain about a future patch that adds
"renesas,pci-r8a7793" to a DT, which helps ensure that shipped DTs use
documented compatibility strings.

[bhelgaas: changelog]
Signed-off-by: Simon Horman <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Acked-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/pci/pci-rcar-gen2.txt

···
 Required properties:
 - compatible: "renesas,pci-r8a7790" for the R8A7790 SoC;
 	      "renesas,pci-r8a7791" for the R8A7791 SoC;
+	      "renesas,pci-r8a7793" for the R8A7793 SoC;
 	      "renesas,pci-r8a7794" for the R8A7794 SoC;
 	      "renesas,pci-rcar-gen2" for a generic R-Car Gen2 compatible device