nrf/drivers/ticker: Reset slow ticker callback count on soft reboot.

The micro:bit board (and probably other boards using the music or display
module) locked up on soft reboot.  Reason was a buffer overflow caused by
an index counter, which was not reset on soft_reboot.

That's fixed in this commit.  Tested with a micro:bit board, performing a
series of soft reboots.

Signed-off-by: robert-hh <robert@hammelrath.com>

ports/nrf/drivers/ticker.c

@@ -62,6 +62,7 @@ void ticker_init0(void) {
 #else
     NRFX_IRQ_PRIORITY_SET(FastTicker_IRQn, 2);
 #endif
+    m_num_of_slow_tickers = 0;
 
     NRFX_IRQ_PRIORITY_SET(SlowTicker_IRQn, 3);