MIPS: make oprofile use cp0_perfcount_irq if it is set

Make the oprofile code use the performance counters irq.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3723/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>