boards/samd21-xpro: enable DAC

samd21-xpro exposes PA02 on the EXT3 pin header, so we can enable
the DAC

boards/samd21-xpro/Makefile.features

@@ -3,6 +3,7 @@ CPU_MODEL = samd21j18a
 
 # Put defined MCU peripherals here (in alphabetical order)
 FEATURES_PROVIDED += periph_adc
+FEATURES_PROVIDED += periph_dac
 FEATURES_PROVIDED += periph_i2c
 FEATURES_PROVIDED += periph_pwm
 FEATURES_PROVIDED += periph_rtc

boards/samd21-xpro/include/periph_conf.h

@@ -347,6 +347,15 @@ static const adc_conf_chan_t adc_channels[] = {
 #define ADC_NUMOF                           ARRAY_SIZE(adc_channels)
 /** @} */
 
+/**
+ * @name DAC configuration
+ * @{
+ */
+#define DAC_CLOCK           SAM0_GCLK_1MHZ
+                            /* use Vcc as reference voltage */
+#define DAC_VREF            DAC_CTRLB_REFSEL_AVCC
+/** @} */
+
 #ifdef __cplusplus
 }
 #endif