dt-bindings: add DT binding for the Aardvark PCIe controller

Add the documentation for the Device Tree binding for the Aardvark PCIe
controller, found on Marvell Armada 3700 ARM64 SoCs.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/devicetree/bindings/pci/aardvark-pci.txt

+Aardvark PCIe controller
+
+This PCIe controller is used on the Marvell Armada 3700 ARM64 SoC.
+
+The Device Tree node describing an Aardvark PCIe controller must
+contain the following properties:
+
+ - compatible: Should be "marvell,armada-3700-pcie"
+ - reg: range of registers for the PCIe controller
+ - interrupts: the interrupt line of the PCIe controller
+ - #address-cells: set to <3>
+ - #size-cells: set to <2>
+ - device_type: set to "pci"
+ - ranges: ranges for the PCI memory and I/O regions
+ - #interrupt-cells: set to <1>
+ - msi-controller: indicates that the PCIe controller can itself
+   handle MSI interrupts
+ - msi-parent: pointer to the MSI controller to be used
+ - interrupt-map-mask and interrupt-map: standard PCI properties to
+   define the mapping of the PCIe interface to interrupt numbers.
+ - bus-range: PCI bus numbers covered
+
+In addition, the Device Tree describing an Aardvark PCIe controller
+must include a sub-node that describes the legacy interrupt controller
+built into the PCIe controller. This sub-node must have the following
+properties:
+
+ - interrupt-controller
+ - #interrupt-cells: set to <1>
+
+Example:
+
+	pcie0: pcie@d0070000 {
+		compatible = "marvell,armada-3700-pcie";
+		device_type = "pci";
+		status = "disabled";
+		reg = <0 0xd0070000 0 0x20000>;
+		#address-cells = <3>;
+		#size-cells = <2>;
+		bus-range = <0x00 0xff>;
+		interrupts = <GIC_SPI 29 IRQ_TYPE_LEVEL_HIGH>;
+		#interrupt-cells = <1>;
+		msi-controller;
+		msi-parent = <&pcie0>;
+		ranges = <0x82000000 0 0xe8000000   0 0xe8000000 0 0x1000000 /* Port 0 MEM */
+			  0x81000000 0 0xe9000000   0 0xe9000000 0 0x10000>; /* Port 0 IO*/
+		interrupt-map-mask = <0 0 0 7>;
+		interrupt-map = <0 0 0 1 &pcie_intc 0>,
+				<0 0 0 2 &pcie_intc 1>,
+				<0 0 0 3 &pcie_intc 2>,
+				<0 0 0 4 &pcie_intc 3>;
+		pcie_intc: interrupt-controller {
+			interrupt-controller;
+			#interrupt-cells = <1>;
+		};
+	};