soc: imx: imx8m-blk-ctrl: add DISP blk-ctrl

This adds the description for the i.MX8MM disp blk-ctrl.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>