MIPS: Lantiq: Fix cp0_perfcount_irq mapping

The introduction of the OF support broke the cp0_perfcount_irq mapping. This
resulted in oprofile not working anymore.

Offending commit is :

commit 3645da02
Author: John Crispin <blogic@openwrt.org>
Date:   Tue Apr 17 10:18:32 2012 +0200

OF: MIPS: lantiq: implement irq_domain support
Signed-off-by: NConor O'Gorman <i@conorogorman.net>
Signed-off-by: NJohn Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4875/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/lantiq/irq.c

@@ -408,7 +408,7 @@ int __init icu_of_init(struct device_node *node, struct device_node *parent)
 #endif
 
 	/* tell oprofile which irq to use */
-	cp0_perfcount_irq = LTQ_PERF_IRQ;
+	cp0_perfcount_irq = irq_create_mapping(ltq_domain, LTQ_PERF_IRQ);
 
 	/*
 	 * if the timer irq is not one of the mips irqs we need to