ARM: dts: r8a7791: Use USB3.0 fallback compatibility string

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

arch/arm/boot/dts/r8a7791.dtsi

@@ -1520,7 +1520,7 @@ msiof2: spi@e6e00000 {
 	};
 
 	xhci: usb@ee000000 {
-		compatible = "renesas,xhci-r8a7791";
+		compatible = "renesas,xhci-r8a7791", "renesas,rcar-gen2-xhci";
 		reg = <0 0xee000000 0 0xc00>;
 		interrupts = <GIC_SPI 101 IRQ_TYPE_LEVEL_HIGH>;
 		clocks = <&mstp3_clks R8A7791_CLK_SSUSB>;