arm64: dts: imx8mp: Fix missing GPC Interrupt

arm64: dts: imx8mp: Fix missing GPC Interrupt

[ Upstream commit b3b75ace2085aca623c57e04ea7218ae690090fb ]

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: fc0f05124621 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>