ARM: dts: msm: Add cooling device property to sdm670 msm gpu

Add cooling-cells property to msm gpu for sdm670. This enables
the gpu cooling device to be referenced in a thermal zone definition.

Change-Id: I12a617ae2edcd71a6ea46f18db9c2c7e0e3a75b0
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>