ARM: dts: r8a7791: use fallback pci compatibility string

Use recently added fallback compatibility string in r8a7791 device tree.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r8a7791.dtsi

@@ -1530,7 +1530,7 @@ xhci: usb@ee000000 {
 	};
 
 	pci0: pci@ee090000 {
-		compatible = "renesas,pci-r8a7791";
+		compatible = "renesas,pci-r8a7791", "renesas,pci-rcar-gen2";
 		device_type = "pci";
 		reg = <0 0xee090000 0 0xc00>,
 		      <0 0xee080000 0 0x1100>;
@@ -1565,7 +1565,7 @@ usb@0,2 {
 	};
 
 	pci1: pci@ee0d0000 {
-		compatible = "renesas,pci-r8a7791";
+		compatible = "renesas,pci-r8a7791", "renesas,pci-rcar-gen2";
 		device_type = "pci";
 		reg = <0 0xee0d0000 0 0xc00>,
 		      <0 0xee0c0000 0 0x1100>;