nrf/drivers/ticker: Rework ticker functions for microbit display/music.

- Rename init function to ticker_init0.
- Implement ticker_register_low_pri_callback (recycle of unused
  set_low_priority_callback function which was unimplemented).
- Add support for registering 2 low pri callbacks.  For now, one intended
  for microbit display, and one for modmusic.

ports/nrf/drivers/ticker.c

@@ -39,11 +39,10 @@
 #define SlowTicker_IRQHandler SWI0_IRQHandler
 
 // Ticker callback function called every MACRO_TICK
-static volatile callback_ptr slow_ticker;
-
-void ticker_init(callback_ptr slow_ticker_callback) {
-    slow_ticker = slow_ticker_callback;
+static volatile uint8_t      m_num_of_slow_tickers = 0;
+static volatile callback_ptr m_slow_tickers[2]     = {NULL, NULL};
 
+void ticker_init0(void) {
     NRF_TIMER_Type *ticker = FastTicker;
 #ifdef NRF51
     ticker->POWER = 1;
@@ -70,6 +69,10 @@ void ticker_init(callback_ptr slow_ticker_callback) {
     hal_irq_enable(SlowTicker_IRQn);
 }
 
+void ticker_register_low_pri_callback(callback_ptr slow_ticker_callback) {
+    m_slow_tickers[m_num_of_slow_tickers++] = slow_ticker_callback;
+}
+
 /* Start and stop timer 1 including workarounds for Anomaly 73 for Timer
 * http://www.nordicsemi.com/eng/content/download/29490/494569/file/nRF51822-PAN%20v3.0.pdf
 */
@@ -156,7 +159,12 @@ int clear_ticker_callback(uint32_t index) {
 
 void SlowTicker_IRQHandler(void)
 {
-    slow_ticker();
+
+    for (int i = 0; i < m_num_of_slow_tickers; i++) {
+        if (m_slow_tickers[i] != NULL) {
+            m_slow_tickers[i]();
+        }
+    }
 }
 
 #endif // MICROPY_PY_MACHINE_SOFT_PWM

ports/nrf/drivers/ticker.h

@@ -10,14 +10,13 @@
 typedef void (*callback_ptr)(void);
 typedef int32_t (*ticker_callback_ptr)(void);
 
-void ticker_init(callback_ptr slow_ticker_callback);
+void ticker_init0();
 void ticker_start(void);
 void ticker_stop(void);
-
 int clear_ticker_callback(uint32_t index);
 int set_ticker_callback(uint32_t index, ticker_callback_ptr func, int32_t initial_delay_us);
 
-int set_low_priority_callback(callback_ptr callback, int id);
+void ticker_register_low_pri_callback(callback_ptr callback);
 
 #define CYCLES_PER_MICROSECONDS 16