irqchip/mtk-cirq: Convert to a platform driver

This driver can work as a platform driver. So covert it to a platform
driver.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Hanks Chen <hanks.chen@mediatek.com>
Link: https://lore.kernel.org/r/20200718000637.3632841-5-saravanak@google.com

drivers/irqchip/irq-mtk-cirq.c

@@ -295,4 +295,6 @@ out_free:
 	return ret;
 }
 
-IRQCHIP_DECLARE(mtk_cirq, "mediatek,mtk-cirq", mtk_cirq_of_init);
+IRQCHIP_PLATFORM_DRIVER_BEGIN(mtk_cirq)
+IRQCHIP_MATCH("mediatek,mtk-cirq", mtk_cirq_of_init)
+IRQCHIP_PLATFORM_DRIVER_END(mtk_cirq)