thermal: Introduce a debugfs-based testing facility

Introduce a facility allowing the thermal core functionality to be
exercised in a controlled way in order to verify its behavior, without
affecting its regular users noticeably.

It is based on the idea of preparing thermal zone templates along with
their trip points by writing to files in debugfs.  When ready, those
templates can be used for registering test thermal zones with the
thermal core.

The temperature of a test thermal zone created this way can be adjusted
via debugfs, which also triggers a __thermal_zone_device_update() call
for it.  By manipulating the temperature of a test thermal zone, one can
check if the thermal core reacts to the changes of it as expected.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/6065927.lOV4Wx5bFT@rjwysocki.net


[ rjw: Fixed ordering of kcalloc() arguments ]
[ rjw: Fixed debugfs_create_dir() return value checks ]
[ rjw: Fixed two kerneldoc comments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>