soc: imx: gpcv2: Add dispmix and mipi domains to imx8mn

The dispmix will be needed for the blkctl driver, so add it
to the gpcv2.

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