[PATCH] powerpc: more g5 overtemp problem fix

Some G5s still occasionally experience shutdowns due to overtemp
conditions despite the recent fix. After analyzing logs from such
machines, it appears that the overtemp code is a bit too quick at
shutting the machine down when reaching the critical temperature (tmax +
8) and doesn't leave the fan enough time to actually cool it down. This
happens if the temperature of a CPU suddenly rises too high in a very
short period of time, or occasionally on boot (that is the CPUs are
already overtemp by the time the driver loads).

This patches makes the code a bit more relaxed, leaving a few seconds to
the fans to do their job before kicking the machine shutown.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>