soc: imx: gpc: keep PGC_X_CTRL name align with reference manual

Instead of GPC_PGC_PDN_OFFS, naming it as GPC_PGC_CTRL_OFFS which is
defined in reference manual for better reading.

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>