MIPS: c-octeon: Provide alternative SMP cache flush function

Currently c-octeon relies on octeon's own smp function to flush
I-Cache. However this function is not available on generic platform.

Just use smp_call_function_many on generic platform.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>