boards/nucleo32-l031: add adc configuration

boards/nucleo32-l031/Makefile.features

 # Put defined MCU peripherals here (in alphabetical order)
+FEATURES_PROVIDED += periph_adc
 FEATURES_PROVIDED += periph_gpio
 FEATURES_PROVIDED += periph_pwm
 FEATURES_PROVIDED += periph_rtc

boards/nucleo32-l031/include/periph_conf.h

@@ -153,7 +153,16 @@ static const spi_conf_t spi_config[] = {
  * @name    ADC configuration
  * @{
  */
-#define ADC_NUMOF           (0)
+#define ADC_CONFIG {                           \
+    { GPIO_PIN(PORT_A, 0), 0 },  /* Pin A0 */  \
+    { GPIO_PIN(PORT_A, 1), 1 },  /* Pin A1 */  \
+    { GPIO_PIN(PORT_A, 3), 3 },  /* Pin A2 */  \
+    { GPIO_PIN(PORT_A, 4), 4 },  /* Pin A3 */  \
+    { GPIO_PIN(PORT_A, 5), 5 },  /* Pin A4 */  \
+    { GPIO_PIN(PORT_A, 6), 6 },  /* Pin A5 */  \
+    { GPIO_PIN(PORT_A, 7), 7 },  /* Pin A6 */  \
+}
+#define ADC_NUMOF           (7U)
 /** @} */
 
 /**