mvebu: LS421DE: use automatic fan control with thermal zones

mvebu: LS421DE: use automatic fan control with thermal zones

The Buffalo Linkstation LS421DE has a chassis fan for cooling two internal
hard drives. Currently there is no control over this fan, running always
at fixed medium speed.

With the recent jump to the kernel 5.4, now we can monitor the hard drive
temperature and control the fan with thermal zones.

Install the kmod-hwmon-drivetemp module and wire up a thermal zone on the
dts file to allow automatic fan control by the kernel.

Tested succesfully using a single Crucial BX500 SSD drive.

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