MIPS: ralink: add missing comment in irq driver

Trivial patch that adds a comment that makes the code more readable.
Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5168/

arch/mips/ralink/irq.c

@@ -166,6 +166,7 @@ static int __init intc_of_init(struct device_node *node,
 	irq_set_chained_handler(irq, ralink_intc_irq_handler);
 	irq_set_handler_data(irq, domain);
 
+	/* tell the kernel which irq is used for performance monitoring */
 	cp0_perfcount_irq = irq_create_mapping(domain, 9);
 
 	return 0;