PCI: Add DT binding for Axis ARTPEC-6 PCIe controller

Add the Device Tree binding documentation that allows to describe the PCIe
controller found in the Axis ARTPEC-6 SoC.
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/axis,artpec6-pcie.txt

+* Axis ARTPEC-6 PCIe interface
+
+This PCIe host controller is based on the Synopsys DesignWare PCIe IP
+and thus inherits all the common properties defined in designware-pcie.txt.
+
+Required properties:
+- compatible: "axis,artpec6-pcie", "snps,dw-pcie"
+- reg: base addresses and lengths of the PCIe controller (DBI),
+	the phy controller, and configuration address space.
+- reg-names: Must include the following entries:
+	- "dbi"
+	- "phy"
+	- "config"
+- interrupts: A list of interrupt outputs of the controller. Must contain an
+  entry for each entry in the interrupt-names property.
+- interrupt-names: Must include the following entries:
+	- "msi": The interrupt that is asserted when an MSI is received
+- axis,syscon-pcie: A phandle pointing to the ARTPEC-6 system controller,
+	used to enable and control the Synopsys IP.
+
+Example:
+
+	pcie@f8050000 {
+		compatible = "axis,artpec6-pcie", "snps,dw-pcie";
+		reg = <0xf8050000 0x2000
+		       0xf8040000 0x1000
+		       0xc0000000 0x1000>;
+		reg-names = "dbi", "phy", "config";
+		#address-cells = <3>;
+		#size-cells = <2>;
+		device_type = "pci";
+			  /* downstream I/O */
+		ranges = <0x81000000 0 0x00010000 0xc0010000 0 0x00010000
+			  /* non-prefetchable memory */
+			  0x82000000 0 0xc0020000 0xc0020000 0 0x1ffe0000>;
+		num-lanes = <2>;
+		interrupts = <GIC_SPI 148 IRQ_TYPE_LEVEL_HIGH>;
+		interrupt-names = "msi";
+		#interrupt-cells = <1>;
+		interrupt-map-mask = <0 0 0 0x7>;
+		interrupt-map = <0 0 0 1 &intc GIC_SPI 144 IRQ_TYPE_LEVEL_HIGH>,
+		                <0 0 0 2 &intc GIC_SPI 145 IRQ_TYPE_LEVEL_HIGH>,
+		                <0 0 0 3 &intc GIC_SPI 146 IRQ_TYPE_LEVEL_HIGH>,
+		                <0 0 0 4 &intc GIC_SPI 147 IRQ_TYPE_LEVEL_HIGH>;
+		axis,syscon-pcie = <&syscon>;
+	};