MIPS: Loongson: Oprofile: add a new do_perfcnt_IRQ()

On FuLoong-2F IP6 is shared by the performance counter overflow interrupt
and the Bonito northbridge interrupt.  To reduce overhead only call
do_IRQ() when oprofile is enabled to reduce overhead.

This patch adds an inline function do_perfcnt_IRQ() to hide the #if's ,
which can be shared by the other Loongson machines, i.e. gdium.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1492/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>