stm class: Add source type

Currently kernel HW tracing infrastrtucture and specifically its SyS-T
part treats all source data in the same way. Treating and encoding
different trace data sources differently might allow decoding software
to make use of e.g. ftrace event ids by converting them to a SyS-T
message catalog.

The solution is to keep source type stored within stm_source_data
structure to allow different handling by stm output/protocol.
Currently we only differentiate between STM_USER and STM_FTRACE sources.

Signed-off-by: Mikhail Lappo <miklelappo@gmail.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240429130119.1518073-3-alexander.shishkin@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>