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

drivers/irqchip/irq-mmp.c

@@ -468,7 +468,12 @@ static int __init mmp3_of_init(struct device_node *node,
 	icu_data[0].conf_disable = mmp3_conf.conf_disable;
 	icu_data[0].conf_mask = mmp3_conf.conf_mask;
 	icu_data[0].conf2_mask = mmp3_conf.conf2_mask;
+
+	if (!parent) {
+		/* This is the main interrupt controller. */
 		set_handle_irq(mmp2_handle_irq);
+	}
+
 	max_icu_nr = 1;
 	return 0;
 }