drm/radeon: expose DPM thermal thresholds through sysfs

The hwmon sysfs interface allows exposing temperature limits. The "max"
and "min" thresholds will be exposed as a critical high limit and its
hysteresis value, respectively. This gives the user a better idea of how
well cooling is doing and whether it is sufficient.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>