boards/nrf51dongle: added dummy ADC config

boards/nrf51dongle/include/periph_conf.h

@@ -80,6 +80,15 @@ static const timer_conf_t timer_config[] = {
 #define UART_PIN_CTS        10
 /** @} */
 
+/**
+ * @brief   ADC configuration
+ *
+ * The configuration consists simply of a list of channels that should be used
+ * @{
+ */
+#define ADC_NUMOF          (0)
+/** @} */
+
 /**
  * @name Radio device configuration
  *