drivers/gpio: added default ISR context struct

drivers/include/periph/gpio.h

@@ -122,6 +122,18 @@ typedef enum {
  */
 typedef void (*gpio_cb_t)(void *arg);
 
+/**
+ * @brief   Default interrupt context for GPIO pins
+ * @{
+ */
+#ifndef HAVE_GPIO_ISR_CTX_T
+typedef struct {
+    gpio_cb_t cb;           /**< interrupt callback */
+    void *arg;              /**< optional argument */
+} gpio_isr_ctx_t;
+#endif
+/** @} */
+
 /**
  * @brief   Initialize the given pin as general purpose input or output
  *