soc: imx: gpc: use GPC_PGC_DOMAIN_* indexes

Use GPC_PGC_DOMAIN_* indexes consistent.

Signed-off-by: Sven Schmitt <sven.schmitt@mixed-mode.de>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>