rp2: Refactor soft timer to use hardware timer alarm.

Progress towards removing pico-sdk alarm pool, due to a known issue.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

shared/runtime/softtimer.h

@@ -81,6 +81,9 @@ static inline void soft_timer_reinsert(soft_timer_entry_t *entry, uint32_t initi
 //   pend-SV IRQ level, or equivalent.
 uint32_t soft_timer_get_ms(void);
 void soft_timer_schedule_at_ms(uint32_t ticks_ms);
+
+// Optional port-specific initialisation function (provided and called by the port if needed).
+void soft_timer_init(void);
 #endif
 
 #endif // MICROPY_INCLUDED_SHARED_RUNTIME_SOFTTIMER_H