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>