powerpc/powermac: Make auto-loading of therm_pm72 possible

The therm_pm72 driver, used on the PowerMac G5 range, cannot be
auto-loaded, since the driver itself creates both the device node
and the driver instance.

Moving the device node creation to the platform setup code and
adding the necessary MODULE_DEVICE_TABLE() information allows the
driver to be automatically loaded by udev on any semi-modern
distribution.

It "fixes" a major source of problem on G5 machines where the
driver wasn't explicitely loaded by default, and the system
would automatically shutdown under load.

Tested on an Xserve G5.

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>