Provide a slightly more-tolerant set of thermal parameters for PowerMac

Provide a slightly more-tolerant set of thermal parameters for PowerMac
motherboard temperatures. In particular, the U4 northbridge die is very
hard to cool or heat effectively with fans and is not responsive to load.
It generally sits around 64C, where it seems happy, so (like Linux) just
declare that to be its target temperature.

This makes the PowerMac G5 much less loud, with no change in the
temperatures of any system components.

MFC after:	2 weeks