thermal: exynos: Add the dependency of CONFIG_THERMAL_OF instead of CONFIG_OF

The exynos thermal driver use the of_thermal_*() API to parse the basic data
for thermal management from devicetree file. So, if CONFIG_EXYNOS_THERMAL is
selected without CONFIG_THERMAL_OF, kernel can build it without any problem.
But, exynos thermal driver is not working with following error log. This patch
add the dependency of CONFIG_THERMAL_OF instead of CONFIG_OF.

[    1.458644] get_th_reg: Cannot get trip points from of-thermal.c!
[    1.459096] get_th_reg: Cannot get trip points from of-thermal.c!
[    1.465211] exynos4412_tmu_initialize: No CRITICAL trip point defined at of-thermal.c!

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>