irqchip/mmp: Coexist with GIC root IRQ controller

On MMP3, the GIC can be set as a root IRQ interrupt controller. If the
device tree indicated that GIC is enabled, avoid hooking up
mmp2_handle_irq().

The interrupt muxes are still being used.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20190822092643.593488-10-lkundrak@v3.sk