dt-bindings: display: imx: fix parallel display interface-pix-fmt property

The parallel display device tree binding documentation incorrectly lists
the interface-pix-fmt property with underscores ("interface_pix_fmt").
This was never supported by any driver, and the DT example in the same
file always contained the correct spelling ("interface-pix-fmt").

See commit 19022aaa ("staging: drm/imx: Add parallel display
support") and commit 2d62da8e ("staging: drm/imx: Add devicetree
binding documentation") for reference.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -116,7 +116,7 @@ Parallel display support
 Required properties:
 - compatible: Should be "fsl,imx-parallel-display"
 Optional properties:
-- interface_pix_fmt: How this display is connected to the
+- interface-pix-fmt: How this display is connected to the
   display interface. Currently supported types: "rgb24", "rgb565", "bgr666"
   and "lvds666".
 - edid: verbatim EDID data block describing attached display.