ARM: dts: Add CPU cooling binding for exynos3250 boards

This patch add the cooling device to control the overheating issue on
Exynos3250-based Rinato/Monk board.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>