gpu: ipu-v3: document valid IPUv3 compatibles and extend for i.MX6 Qu…

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Document the valid compatible strings for the IPUv3.

On i.MX6 QuadPlus the IPU needs to know which PRG has to be
used for this IPU instance. Add a "fsl,prg" property containing
a phandle pointing to the correct PRG device.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/display/imx/fsl-imx-drm.txt

@@ -21,13 +21,19 @@ Freescale i.MX IPUv3
 ====================
 
 Required properties:
-- compatible: Should be "fsl,<chip>-ipu"
+- compatible: Should be "fsl,<chip>-ipu" where <chip> is one of
+  - imx51
+  - imx53
+  - imx6q
+  - imx6qp
 - reg: should be register base and length as documented in the
   datasheet
 - interrupts: Should contain sync interrupt and error interrupt,
   in this order.
 - resets: phandle pointing to the system reset controller and
           reset line index, see reset/fsl,imx-src.txt for details
+Additional required properties for fsl,imx6qp-ipu:
+- fsl,prg: phandle to prg node associated with this IPU instance
 Optional properties:
 - port@[0-3]: Port nodes with endpoint definitions as defined in
   Documentation/devicetree/bindings/media/video-interfaces.txt.