thermal: Move trip point structure definition to private header

The struct thermal_trip is only used by the thermal internals, it is
pointless to export the definition in the global header.

Move the structure to the thermal_core.h internal header.

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/20200402142747.8307-4-daniel.lezcano@linaro.org