mvebu: LS421DE: fix the thermal zones

mvebu: LS421DE: fix the thermal zones

The thermal zones kernel documentation is misleading, we cannot use more
than one sensor in a thermal zone node.

Furthermore the drivetemp driver for some reason it only catches one
sensor from the hard drives array (the first available).

In the Buffalo Linkstation LS421DE board there is also a sensor at the
ethernet phy chip that can also be monitored. Very useful to stop the fan
when there are no hard drives in the bays.

(It might be also possible to add the CPU sensor, but it requires kernel
patching for registering the sensor via device tree, using the function:
devm_thermal_zone_of_sensor_register)

Fix the thermal zones to use only one sensor per node and add the ethernet
phy sensor. Also adjust the hdd temperatures to be more conservative for
a mechanical hard drive.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>