kirkwood: use 3 temperature sensors for Zyxel NSA310B

Instead of taking the input of one temperature sensor (temp1), the
script takes into account three temperature sensors to control the
PWM of the cooling fan.

temp1 -> placed on main board
temp2 -> placed on main board
temp3 -> placed on or close to chipset

All three temperatures give valid input for the PWM of the fan on
NSA310 and are actually changing.

Tested on two NSA310.

Signed-off-by: Thomas Beckler <thomas.beckler@hotmail.com>
Reviewed-by: Alberto Bursi <bobafetthotmail@gmail.com>
[commit title/message facelift, code cleanup]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>