topology2: doc: Little fixes to make all classes show correctly

There were couple of classes that did not have their topology2 snippets
showing. Probably the essential problem was just the class definition
starting from the first line of the file. There was nothing obviously
wrong in the output of the filter, but the way the doxygen works is
sometimes hard to anticipate. Adding \struct before the definition
fixed the problem.

Signed-off-by: Jyri Sarha <jyri.sarha@linux.intel.com>

Author: Jyri Sarha

tools/topology/topology2/include/dais/mic_extension.conf

@@ -1,3 +1,6 @@
+## \brief Mic extension class
+## \struct mic_extension
+
 Class.Base."mic_extension" {
 
 	DefineAttribute."id" {}

tools/topology/topology2/include/dais/pdm_config.conf

@@ -1,3 +1,6 @@
+## \brief Class for PDM config
+## \struct pdm_config
+
 Class.Base."pdm_config" {
 	DefineAttribute."instance" {}
 	#

tools/topology/topology2/platform/intel/ssp_aux_config.conf

@@ -1,3 +1,5 @@
+## \struct mn_config
+
 Class.Base."mn_config" {
 	DefineAttribute."id" {}