shared/runtime/softtimer: Generalise soft_timer to work without SysTick.

If a port defines MICROPY_SOFT_TIMER_TICKS_MS then soft_timer assumes a
SysTick back end, and provides a soft_timer_next variable that sets when
the next call to soft_timer_handler() should occur.

Otherwise, a port should provide soft_timer_get_ms() and
soft_timer_schedule_at_ms() with appropriate semantics (see comments).

Existing users of soft_timer should continue to work as they did.

Signed-off-by: Damien George <damien@micropython.org>

ports/samd/mphalport.h

@@ -35,6 +35,9 @@
 #include "hpl_time_measure.h"
 #include "sam.h"
 
+#define MICROPY_PY_PENDSV_ENTER   uint32_t atomic_state = raise_irq_pri(IRQ_PRI_PENDSV)
+#define MICROPY_PY_PENDSV_EXIT    restore_irq_pri(atomic_state)
+
 #define MICROPY_HW_USB_CDC_TX_TIMEOUT (500)
 
 extern int mp_interrupt_char;