Add ticker_free() function to the ticker interface.

hal/mbed_lp_ticker_api.c

@@ -35,6 +35,7 @@ static const ticker_interface_t lp_interface = {
 #endif
     .fire_interrupt = lp_ticker_fire_interrupt,
     .get_info = lp_ticker_get_info,
+    .free = lp_ticker_free,
 };
 
 static const ticker_data_t lp_data = {

hal/mbed_us_ticker_api.c

@@ -27,6 +27,7 @@ static const ticker_interface_t us_interface = {
     .set_interrupt = us_ticker_set_interrupt,
     .fire_interrupt = us_ticker_fire_interrupt,
     .get_info = us_ticker_get_info,
+    .free = us_ticker_free,
 };
 
 static const ticker_data_t us_data = {

hal/ticker_api.h

@@ -64,6 +64,7 @@ typedef struct {
     void (*clear_interrupt)(void);                /**< Clear interrupt function */
     void (*set_interrupt)(timestamp_t timestamp); /**< Set interrupt function */
     void (*fire_interrupt)(void);                 /**< Fire interrupt right-away */
+    void (*free)(void);                           /**< Disable function */
     const ticker_info_t *(*get_info)(void);       /**< Return info about this ticker's implementation */
 } ticker_interface_t;