arm64: dts: imx8mm: add DISP blk-ctrl

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

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>