* Open PIC Binding

This binding specifies what properties must be available in the device tree
representation of an Open PIC compliant interrupt controller.  This binding is
based on the binding defined for Open PIC in [1] and is a superset of that
binding.

Required properties:

  NOTE: Many of these descriptions were paraphrased here from [1] to aid
        readability.

    - compatible: Specifies the compatibility list for the PIC.  The type
      shall be <string> and the value shall include "open-pic".

    - reg: Specifies the base physical address(s) and size(s) of this
      PIC's addressable register space.  The type shall be <prop-encoded-array>.

    - interrupt-controller: The presence of this property identifies the node
      as an Open PIC.  No property value shall be defined.

    - #interrupt-cells: Specifies the number of cells needed to encode an
      interrupt source.  The type shall be a <u32> and the value shall be 2.

    - #address-cells: Specifies the number of cells needed to encode an
      address.  The type shall be <u32> and the value shall be 0.  As such,
      'interrupt-map' nodes do not have to specify a parent unit address.

Optional properties:

    - pic-no-reset: The presence of this property indicates that the PIC
      shall not be reset during runtime initialization.  No property value shall
      be defined.  The presence of this property also mandates that any
      initialization related to interrupt sources shall be limited to sources
      explicitly referenced in the device tree.

* Interrupt Specifier Definition

  Interrupt specifiers consists of 2 cells encoded as
  follows:

    - <1st-cell>: The interrupt-number that identifies the interrupt source.

    - <2nd-cell>: The level-sense information, encoded as follows:
                    0 = low-to-high edge triggered
                    1 = active low level-sensitive
                    2 = active high level-sensitive
                    3 = high-to-low edge triggered

* Examples

Example 1:

	/*
	 * An Open PIC interrupt controller
	 */
	mpic: pic@40000 {
		// This is an interrupt controller node.
		interrupt-controller;

		// No address cells so that 'interrupt-map' nodes which reference
		// this Open PIC node do not need a parent address specifier.
		#address-cells = <0>;

		// Two cells to encode interrupt sources.
		#interrupt-cells = <2>;

		// Offset address of 0x40000 and size of 0x40000.
		reg = <0x40000 0x40000>;

		// Compatible with Open PIC.
		compatible = "open-pic";

		// The PIC shall not be reset.
		pic-no-reset;
	};

Example 2:

	/*
	 * An interrupt generating device that is wired to an Open PIC.
	 */
	serial0: serial@4500 {
		// Interrupt source '42' that is active high level-sensitive.
		// Note that there are only two cells as specified in the interrupt
		// parent's '#interrupt-cells' property.
		interrupts = <42 2>;

		// The interrupt controller that this device is wired to.
		interrupt-parent = <&mpic>;
	};

* References

[1] Devicetree Specification
    (https://www.devicetree.org/specifications/)