doc: add all cm3 headers to doxygen.

Just get them all included as they are.  Gets them grouped up and
visible so we can start clarifying the rest of their docs.

include/libopencm3/cm3/dwt.h

@@ -20,6 +20,12 @@
 #ifndef LIBOPENCM3_CM3_DWT_H
 #define LIBOPENCM3_CM3_DWT_H
 
+/**
+ * @defgroup cm_fpb Cortex-M Flash Patch and Breakpoint (FPB) unit
+ * @ingroup CM3_defines
+ * @{
+ */
+
 #include <libopencm3/cm3/common.h>
 #include <libopencm3/cm3/memorymap.h>
 
@@ -149,4 +155,6 @@ uint32_t dwt_read_cycle_counter(void);
 
 END_DECLS
 
+/**@}*/
+
 #endif /* LIBOPENCM3_CM3_DWT_H */