thermal: core: Get thermal zone by id

The next patch will introduce the generic netlink protocol to handle
events, sampling and command from the thermal framework. In order to
deal with the thermal zone, it uses its unique identifier to
characterize it in the message. Passing an integer is more efficient
than passing an entire string.

This change provides a function returning back a thermal zone pointer
corresponding to the identifier passed as parameter.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200706105538.2159-2-daniel.lezcano@linaro.org