drm/etnaviv: add devicetree bindings

Etnaviv follows the same priciple as imx-drm to have a virtual
master device node to bind all the individual GPU cores together
into one DRM device.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/display/etnaviv/etnaviv-drm.txt

+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>;
+};