dt-bindings: display: bridge: Add i.MX8qm/qxp pixel combiner binding

This patch adds bindings for i.MX8qm/qxp pixel combiner.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220611141421.718743-4-victor.liu@nxp.com