arm64: dts: marvell: Use a SoC-specific compatible for xHCI on Armada37xx

Even if the Armada 37xx does not any specific setup, the device tree
binding documentation requires to use a SoC-specific version
corresponding to the platform first followed by the generic version.

This patch introduce this new compatible string and updates the
documentation accordingly.
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>

Documentation/devicetree/bindings/usb/usb-xhci.txt

@@ -4,6 +4,7 @@ Required properties:
   - compatible: should be one or more of
 
     - "generic-xhci" for generic XHCI device
+    - "marvell,armada3700-xhci" for Armada 37xx SoCs
     - "marvell,armada-375-xhci" for Armada 375 SoCs
     - "marvell,armada-380-xhci" for Armada 38x SoCs
     - "renesas,xhci-r8a7790" for r8a7790 SoC

arch/arm64/boot/dts/marvell/armada-37xx.dtsi

@@ -106,7 +106,8 @@
 			};
 
 			usb3: usb@58000 {
-				compatible = "generic-xhci";
+				compatible = "marvell,armada3700-xhci",
+				"generic-xhci";
 				reg = <0x58000 0x4000>;
 				interrupts = <GIC_SPI 15 IRQ_TYPE_LEVEL_HIGH>;
 				status = "disabled";