mips: convert to generic helpers for IPI function calls

This converts mips to use the new helpers for smp_call_function() and
friends, and adds support for smp_call_function_single(). Not tested,
but it compiles.

mips shares the same IPI for smp_call_function() and
smp_call_function_single(), since not all mips platforms have enough
available IPIs to support seperate setups.

Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>