drm/amdgpu: Add new AMDGPU_PP_SENSOR_MIN/MAX_FAN_RPM sensor

For getting the min/max fan speed in RPM units.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>