doc: efm32: dac: enable peripheral_apis documentation

lib/efm32/common/dac_common.c

@@ -1,3 +1,6 @@
+/** @addtogroup dac_file DAC peripheral API
+ * @ingroup peripheral_apis
+ */
 /*
  * This file is part of the libopencm3 project.
  *
@@ -22,6 +25,8 @@
 
 #include <libopencm3/efm32/dac.h>
 
+/**@{*/
+
 /**
  * Set DAC refresh cycle
  * @param[in] dac DAC (use DACx)
@@ -166,3 +171,5 @@ void dac_disable_channel(uint32_t dac, enum dac_ch ch)
 {
 	DAC_CHx_CTRL(dac, ch) &= ~DAC_CH_CTRL_REFREN;
 }
+
+/**@}*/