soc: imx: gpcv2: add lockdep annotation

Some of the GPCv2 power domains are nested inside each other without
visibility to lockdep at the genpd level, as they are in separate
driver instances and don't have a parent/child power-domain relationship.

Add a subclass annotation to the nested domains to let lockdep know that
it is okay to take the genpd lock in a nested fashion.

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>