soc: imx: gpc: build gpc only if hardware has gpc

Add CONFIG_HAVE_IMX_GPC to goal definition of gpc.o,
to avoid building it for hardware that does not need it.

Signed-off-by: Marc Ohlf <ohlf@mkt-sys.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>