soc: imx: gpcv2: add PGC control register indirection

The PGC control registers in the shared (not per-PGC) region of the
GPC address space have different offsets on i.MX8MP to make space for
additional interrupt control registers.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>