cpu/sam0_common: added peripheral FREQM configuration declaration

cpu/sam0_common/include/periph_cpu_common.h

@@ -945,6 +945,14 @@ typedef struct {
  */
 #define WDT_HAS_INIT                   (1)
 
+/**
+ * @brief   Frequency meter configuration
+ */
+typedef struct {
+    gpio_t pin;             /**< GPIO at which the frequency is to be measured */
+    uint8_t gclk_src;       /**< GCLK source select for reference */
+} freqm_config_t;
+
 #if defined(REV_DMAC) || DOXYGEN
 /**
  * @name sam0 DMA peripheral