arm64: dts: imx8mp: Fix missing GPC Interrupt

The GPC node references an interrupt parent, but it doesn't
state the interrupt itself.  According to the TRM, this IRQ
is 87. This also eliminate an error detected from dt_binding_check

Fixes: fc0f0512 ("arm64: dts: imx8mp: add GPC node with GPU power domains")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>