[MIPS] Assume R4000/R4400 newer than 3.0 don't have the mfc0 count bug

This seems as reasonable assumption and gets some SNI machines to work
which currently must rely on the cp0 counter as clocksource.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>