gpio: Add Netlogic XLP GPIO devicetree bindings documentation

Netlogic XLP Family GPIO

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This GPIO driver is used for following Netlogic XLP SoCs:

	XLP832, XLP316, XLP208, XLP980, XLP532

Required properties:

-------------------

- compatible: Should be one of the following:

  - "netlogic,xlp832-gpio": For Netlogic XLP832

  - "netlogic,xlp316-gpio": For Netlogic XLP316

  - "netlogic,xlp208-gpio": For Netlogic XLP208

  - "netlogic,xlp980-gpio": For Netlogic XLP980

  - "netlogic,xlp532-gpio": For Netlogic XLP532

- reg: Physical base address and length of the controller's registers.

- #gpio-cells: Should be two. The first cell is the pin number and the second

  cell is used to specify optional parameters (currently unused).

- gpio-controller: Marks the device node as a GPIO controller.

- nr-gpios: Number of GPIO pins supported by the controller.

- interrupt-cells: Should be two. The first cell is the GPIO Number. The

  second cell is used to specify flags. The following subset of flags is

  supported:

  - trigger type:

	1 = low to high edge triggered.

	2 = high to low edge triggered.

	4 = active high level-sensitive.

	8 = active low level-sensitive.

- interrupts: Interrupt number for this device.

- interrupt-parent: phandle of the parent interrupt controller.

- interrupt-controller: Identifies the node as an interrupt controller.

Example:

	gpio: xlp_gpio@34000 {

		compatible = "netlogic,xlp316-gpio";

		reg = <0 0x34100 0x1000

		       0 0x35100 0x1000>;

		#gpio-cells = <2>;

		gpio-controller;

		nr-gpios = <57>;

		#interrupt-cells = <2>;

		interrupt-parent = <&pic>;

		interrupts = <39>;

		interrupt-controller;

	};