stm class: Document the stm_source interface

This patch adds a description of the stm_source device class, an
interface for the kernel code to send traces over STM devices.

Cc: linux-api@vger.kernel.org
Cc: Chunyan Zhang <zhang.chunyan@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>