drm/etnaviv: add devicetree bindings

Etnaviv DRM master device

=========================

The Etnaviv DRM master device is a virtual device needed to list all

Vivante GPU cores that comprise the GPU subsystem.

Required properties:

- compatible: Should be one of

    "fsl,imx-gpu-subsystem"

    "marvell,dove-gpu-subsystem"

- cores: Should contain a list of phandles pointing to Vivante GPU devices

example:

gpu-subsystem {

	compatible = "fsl,imx-gpu-subsystem";

	cores = <&gpu_2d>, <&gpu_3d>;

};

Vivante GPU core devices

========================

Required properties:

- compatible: Should be "vivante,gc"

  A more specific compatible is not needed, as the cores contain chip

  identification registers at fixed locations, which provide all the

  necessary information to the driver.

- reg: should be register base and length as documented in the

  datasheet

- interrupts: Should contain the cores interrupt line

- clocks: should contain one clock for entry in clock-names

  see Documentation/devicetree/bindings/clock/clock-bindings.txt

- clock-names:

   - "bus":    AXI/register clock

   - "core":   GPU core clock

   - "shader": Shader clock (only required if GPU has feature PIPE_3D)

Optional properties:

- power-domains: a power domain consumer specifier according to

  Documentation/devicetree/bindings/power/power_domain.txt

example:

gpu_3d: gpu@00130000 {

	compatible = "vivante,gc";

	reg = <0x00130000 0x4000>;

	interrupts = <0 9 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&clks IMX6QDL_CLK_GPU3D_AXI>,

	         <&clks IMX6QDL_CLK_GPU3D_CORE>,

	         <&clks IMX6QDL_CLK_GPU3D_SHADER>;

	clock-names = "bus", "core", "shader";

	power-domains = <&gpc 1>;

};