arm64: dts: imx8mp: add GPC node with GPU power domains

Add the power domains for the GPUs, which do not require any interaction with
a blk-ctrl, but are simply two PU domains nested inside a MIX domain.

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