stm class: stm_console: Add kernel-console-over-stm driver

This is a simple stm_source class device driver (kernelspace stm trace
source) that registers a console and sends kernel messages over STM
devices.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>