dt-bindings: display: imx: Add i.MX8qxp Display Controller pixel engine

i.MX8qxp Display Controller pixel engine consists of all processing units
that operate in the AXI bus clock domain.  Command sequencer and interrupt
controller of the Display Controller work with AXI bus clock, but they are
not in pixel engine.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250414035028.1561475-5-victor.liu@nxp.com