ARM: dts: exynos: Add CPU cooling in SMDK4412

Add missing cooling devices for CPU thermal zones in Exynos4412 SMDK4412
board.  This allows to scale down CPU frequency (and voltage) in case of
thermal pressure.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200830135200.24304-29-krzk@kernel.org