powerpc/powermac: New windfarm driver for PowerMac G5 (AGP) and Xserve G5

This replaces the old therm_pm72 using the same windfarm infrastructure
that was used for other PowerMac G5 models. The fan speeds and sensors
should now be visible in the same location in sysfs.

The driver is split into separate core modules for PowerMac7,2 (and 7,3)
and RackMac3,1, with a lot of the shared code now in the separate sensor
and control modules.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>