dt-bindings: soc: add binding for i.MX8MN DISP blk-ctrl

Add the DT binding for the i.MX8MN DISP blk-ctrl.

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