dt-bindings: power: imx8mn: add defines for DISP blk-ctrl domains

This adds the defines for the power domains provided by the DISP
blk-ctrl.

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>