irqchip/imx-gpcv2: Mark imx_gpcv2_instance with __ro_after_init

imx_gpcv2_instance will not be updated after init, so mark it with
__ro_after_init.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211214084711.1357325-1-peng.fan@oss.nxp.com