thermal: exynos: Clean up non-DT remnants

Commit 1cd1ecb6

 ("thermal: exynos: Remove non DT based support")
cleaned up some non-DT code. However, there were few more things
needed for complete cleanup to make this driver DT only.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>