drm/nouveau/hwmon: expose the auto_point and pwm_min/max attrs

This patch creates a special group attributes for attrs like "*auto_point*".
We check if we have support for them, and if we do, we gather them all in
an attribute_group's structure which is the parameter regarding special groups
of hwmon_device_register_with_info
We also do the same for pwm_min/max attrs.

Signed-off-by: Oscar Salvador <osalvador.vilardaga@gmail.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>