arm64: dts: imx8mn: add DISP blk-ctrl

Add the DT node for the DISP blk-ctrl. With this in place the
display/mipi power domains should be functional.

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