MIPS: Loongson: Add oprofile support

This kernel support is needed by the user-space tool:oprofile to profile
linux kernel or applications via loongson2 performance counters. you can
enable this driver via CONFIG_OPROFILE = y or m.

On Loongson2 there are two performance counters, each one can count 16
events respectively. when anyone of the performance counter overflows, an
interrupt will be generated and is routed to the IRQ MIPS_CPU_IRQ_BASE + 6.

Signed-off-by: Yanhua <yanh@lemote.com>
Signed-off-by: Wu Zhangjin <wuzj@lemote.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>