[MIPS] Probe for usability of cp0 compare interrupt.

Some processors offer the option of using the interrupt on which
normally the count / compare interrupt would be signaled as a normal
interupt pin.  Previously this required some ugly hackery for each
system which is much easier done by a quick and simple probe.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>