arm64: dts: imx8mq: properly describe IRQ hierarchy

The GPCv2 sits between most of the peripherals and the GIC and
functions as a wakeup controller for the CPU cores.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>