Add lp/us ticker free() function implementation for NRF5

hal/lp_ticker_api.h

@@ -41,6 +41,7 @@ extern "C" {
  *
  * # Undefined behavior
  * * See the @ref hal_ticker_shared "ticker specification"
+ * * Calling any function other than lp_ticker_init after calling lp_ticker_free
  *
  * @see hal_lp_ticker_tests
  *

hal/us_ticker_api.h

@@ -78,6 +78,7 @@ extern "C" {
  * * Calling any function other than ticker_init before the initialization of the ticker
  * * Whether ticker_irq_handler is called a second time if the time wraps and matches the value set by ticker_set_interrupt again
  * * Calling ticker_set_interrupt with a value that has more than the supported number of bits
+ * * Calling any function other than us_ticker_init after calling us_ticker_free
  *
  * # Potential bugs
  * * Drift due to reschedule - Verified by ::ticker_repeat_reschedule_test

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF51/lp_ticker.c

@@ -25,6 +25,12 @@ void lp_ticker_init(void)
     common_rtc_init();
 }
 
+void lp_ticker_free(void)
+{
+    // A common counter is used for RTC, lp_ticker and us_ticker, so it can't be
+    // disabled here, but this does not cause any extra cost.
+}
+
 uint32_t lp_ticker_read()
 {
     return (uint32_t)common_rtc_64bit_us_get();

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF51/us_ticker.c

@@ -266,6 +266,12 @@ void us_ticker_init(void)
     common_rtc_init();
 }
 
+void us_ticker_free(void)
+{
+    // A common counter is used for RTC, lp_ticker and us_ticker, so it can't be
+    // disabled here, but this does not cause any extra cost.
+}
+
 uint32_t us_ticker_read()
 {
     us_ticker_init();