soc: imx: gpcv2: add support for i.MX8MM power domains

This adds support for the power domains found on i.MX8MM. The 2D and 3D
GPU domains are abstracted as a single domain in the driver, as they can't
be powered up/down individually due to a shared reset.

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