ARM: dts: gemini: Add thermal zone to DIR-685

The DIR-685 can now exploit the thermal zone added by the
drive temperature sensor inside the hard drive. We have
patched the libata subsystem to assign the device nodes
properly to the SCSI devices and this is what the drivetemp
driver will use to populate the sensor and the thermal
zone, so pick that up into the thermal zone and let this
control the fan.

The hardware lacks an embedded temperature sensor so the
D-Link vendor firmware uses this method to control the
temperature of the NAS enclosure using the thermal sensor
inside the hard drive.

The drive temperature trigger points to be used comes from
the vendor firmware.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>