thermal: core: Add helpers to browse the cdev, tz and governor list

The cdev, tz and governor list, as well as their respective locks are
statically defined in the thermal_core.c file.

In order to give a sane access to these list, like browsing all the
thermal zones or all the cooling devices, let's define a set of
helpers where we pass a callback as a parameter to be called for each
thermal entity.

We keep the self-encapsulation and ensure the locks are correctly
taken when looking at the list.

Acked-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200706105538.2159-1-daniel.lezcano@linaro.org