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

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.