cpu: fixed any doxygen warning realated to 'periph'

cpu/kinetis_common/include/periph_cpu.h

@@ -38,10 +38,15 @@ typedef uint16_t gpio_t;
  */
 #define GPIO_UNDEF          (0xffff)
 
+/**
+ * @brief   Definition of pin and port positions in the gpio_t type
+ * @{
+ */
 #define GPIO_PORT_SHIFT     (8)
 #define GPIO_PORT_MASK      (0xff << (GPIO_PORT_SHIFT))
 #define GPIO_PIN_SHIFT      (0)
 #define GPIO_PIN_MASK       (0xff << (GPIO_PIN_SHIFT))
+/** @} */
 
 /**
  * @brief   Define a CPU specific GPIO pin generator macro

cpu/samd21/include/periph_cpu.h

@@ -92,7 +92,7 @@ typedef struct {
  * @brief   PWM device configuration data structure
  */
 typedef struct {
-    Tcc *dev;                   /*< TCC device to use */
+    Tcc *dev;                   /**< TCC device to use */
     pwm_conf_chan_t chan[2];    /**< channel configuration */
 } pwm_conf_t;