therm_windtunnel: Convert to a new-style i2c driver

The legacy i2c binding model is going away soon, so convert the ppc
therm_windtunnel driver to the new model or it will break.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>