thermal: exynos: always check for trips points existence

* Check for trip points existence in exynos_tmu_initialize() so it is
  checked on all SoCs.

* Use dev_err() instead of pr_err().

* Fix dev_err() to reference "device tree" not "of-thermal.c".

* Remove no longer needed checks from exynos4210_tmu_initialize() and
  get_th_reg().

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>