thermal: exynos: Provide thermal_exynos.h file to be included in device tree files

This patch is a preparatory patch to be able to read Exynos thermal
configuration from the device tree.

It turned out that DTC is not able to interpret enums properly and hence
it is necessary to #define those values explicitly.

For this reason the ./include/dt-bindings/thermal/thermal_exynos.h file
has been introduced.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>