hwmon: emc2305: Add calls to initialise of cooling maps

Commit 46ef9d4e ("hwmon: emc2305: fixups for driver submitted to
mailing lists") missed adding the call to thermal_of_cooling_device_register
required to configure any cooling maps for the device, hence stopping it
from actually ever changing speed.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>