[POWERPC] windfarm: Add PowerMac 12,1 support

This implements a new driver named windfarm_pm121, which drives the
fans on PowerMac 12,1 machines : iMac G5 iSight (rev C) 17" and
20".  It's based on the windfarm_pm81 driver from Benjamin
Herrenschmidt.

This includes fixes from David Woodhouse correcting the names of some
of the sensors.

Signed-off-by: Étienne Bersac <bersace@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>