thermal: sti: Introduce ST Thermal core code

This core is shared by both ST's 'memory mapped' and
'system configuration register' based Thermal controllers.

Signed-off-by: Ajit Pal Singh <ajitpal.singh@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>