stm class: Document the stm_ftrace

This patch adds a description to the stm_ftrace device source, an
interface for collecting Ftrace's function trace information via
STM devices.

Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
1 file changed