MediaTek MT7621 PCIe controller

Required properties:
- compatible: "mediatek,mt7621-pci"
- device_type: Must be "pci"
- reg: Base addresses and lengths of the PCIe subsys and root ports.
- bus-range: Range of bus numbers associated with this controller.
- #address-cells: Address representation for root ports (must be 3)
- pinctrl-names : The pin control state names.
- pinctrl-0: The "default" pinctrl state.
- #size-cells: Size representation for root ports (must be 2)
- ranges: Ranges for the PCI memory and I/O regions.
- #interrupt-cells: Must be 1
- interrupt-map-mask and interrupt-map: Standard PCI IRQ mapping properties.
  Please refer to the standard PCI bus binding document for a more detailed
  explanation.
- status: either "disabled" or "okay".
- resets: Must contain an entry for each entry in reset-names.
  See ../reset/reset.txt for details.
- reset-names: Must be "pcie0", "pcie1", "pcieN"... based on the number of
  root ports.
- clocks: Must contain an entry for each entry in clock-names.
  See ../clocks/clock-bindings.txt for details.
- clock-names: Must be "pcie0", "pcie1", "pcieN"... based on the number of
  root ports.
- reset-gpios: GPIO specs for the reset pins.

In addition, the device tree node must have sub-nodes describing each PCIe port
interface, having the following mandatory properties:

Required properties:
- reg: Only the first four bytes are used to refer to the correct bus number
      and device number.
- #address-cells: Must be 3
- #size-cells: Must be 2
- ranges: Sub-ranges distributed from the PCIe controller node. An empty
  property is sufficient.
- bus-range: Range of bus numbers associated with this port.

Example for MT7621:

	pcie: pcie@1e140000 {
		compatible = "mediatek,mt7621-pci";
        reg = <0x1e140000 0x100    /* host-pci bridge registers */
               0x1e142000 0x100    /* pcie port 0 RC control registers */
               0x1e143000 0x100    /* pcie port 1 RC control registers */
               0x1e144000 0x100>;  /* pcie port 2 RC control registers */

		#address-cells = <3>;
		#size-cells = <2>;

		pinctrl-names = "default";
		pinctrl-0 = <&pcie_pins>;

		device_type = "pci";

		bus-range = <0 255>;
		ranges = <
			0x02000000 0 0x00000000 0x60000000 0 0x10000000 /* pci memory */
			0x01000000 0 0x00000000 0x1e160000 0 0x00010000 /* io space */
		>;

		#interrupt-cells = <1>;
		interrupt-map-mask = <0xF0000 0 0 1>;
		interrupt-map = <0x10000 0 0 1 &gic GIC_SHARED 4 IRQ_TYPE_LEVEL_HIGH>,
				<0x20000 0 0 1 &gic GIC_SHARED 24 IRQ_TYPE_LEVEL_HIGH>,
				<0x30000 0 0 1 &gic GIC_SHARED 25 IRQ_TYPE_LEVEL_HIGH>;

		status = "disabled";

		resets = <&rstctrl 24 &rstctrl 25 &rstctrl 26>;
		reset-names = "pcie0", "pcie1", "pcie2";
		clocks = <&clkctrl 24 &clkctrl 25 &clkctrl 26>;
		clock-names = "pcie0", "pcie1", "pcie2";

		reset-gpios = <&gpio 19 GPIO_ACTIVE_LOW>,
				<&gpio 8 GPIO_ACTIVE_LOW>,
				<&gpio 7 GPIO_ACTIVE_LOW>;

		pcie@0,0 {
			reg = <0x0000 0 0 0 0>;
			#address-cells = <3>;
			#size-cells = <2>;
			ranges;
			bus-range = <0x00 0xff>;
		};

		pcie@1,0 {
			reg = <0x0800 0 0 0 0>;
			#address-cells = <3>;
			#size-cells = <2>;
			ranges;
			bus-range = <0x00 0xff>;
		};

		pcie@2,0 {
			reg = <0x1000 0 0 0 0>;
			#address-cells = <3>;
			#size-cells = <2>;
			ranges;
			bus-range = <0x00 0xff>;
		};
	};