[MIPS] Add smp_call_function_single()

In the other archs, there is more factoring of smp call code, and more care
in the use of get_cpu(). That can be a follow-up MIPS patch.

Signed-off-by: Peter Watkins <pwatkins@sicortex.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>