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 3645da0276ae9f6938ff29b13904b803ecb68424
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: Conor O'Gorman <i@conorogorman.net>
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4875/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>