Thermal: exynos: Add clk_{un}prepare APIs

clk_{un}prepare APIs are required to migrate to common
clock framework. While at it convert to use devm_clk_get as
it removes some cleanup code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Amit Daniel Kachhap <amit.daniel@samsung.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>