dt-bindings: display: imx: Describe drm binding for fsl,imx-lcdc

dt-bindings: display: imx: Describe drm binding for fsl,imx-lcdc

Modify the existing (fb-like) binding to support the drm-like binding in
parallel.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230306115249.2223042-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230306115249.2223042-2-u.kleine-koenig@pengutronix.de