thermal/of: Move cooling device OF helpers out of thermal core

thermal/of: Move cooling device OF helpers out of thermal core

The functions:
 - thermal_of_cooling_device_register()
 - devm_thermal_of_cooling_device_register()

are specific to device tree usage but are currently implemented in
thermal_core.c.

Move them to thermal_of.c to better reflect the separation between
generic thermal core code and OF-specific logic.

This change is enabled by the recent split of the cooling device
registration into allocation and addition phases, allowing OF-specific
handling (such as device node assignment) to be isolated from the core.

No functional change intended.

Signed-off-by: Daniel Lezcano <daniel.lezcano@oss.qualcomm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@kernel.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Link: https://patch.msgid.link/20260526140802.1059293-17-daniel.lezcano@oss.qualcomm.com