windfarm: Convert to new-style i2c drivers

The legacy i2c binding model is going away soon, so convert the
macintosh windfarm drivers to the new model or they will break.

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