ARM: dts: r8a7791: use fallback pcie 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

@@ -1600,7 +1600,7 @@ usb@0,2 {
 	};
 
 	pciec: pcie@fe000000 {
-		compatible = "renesas,pcie-r8a7791";
+		compatible = "renesas,pcie-r8a7791", "renesas,pcie-rcar-gen2";
 		reg = <0 0xfe000000 0 0x80000>;
 		#address-cells = <3>;
 		#size-cells = <2>;