ARM: dts: exynos: Add cooling levels for Exynos5420 CPUs

On Exynos5420 we support 8 cpufreq steps (600-1300 MHz) for LITTLE and
12 steps for big core (700-1800 MHz). Add respective cooling cells.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>