soc: imx: gpcv2: add domain option to keep domain clocks enabled

Some of the MIX domains are using clocks to drive the bus bridges. Those
must be enabled at all times, as long as the domain is powered up and
they don't have any other consumer than the power domain. Add an option
to keep the clocks attached to a domain enabled as long as the domain
is power up and only disable them after the domain is powered down.

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>