rp2: Remove 1ms timeout to make idle waiting tickless.

The main motivation for doing this was to reduce the latency when the
system is woken by a USB interrupt.  The best_effort_wfe_or_timeout()
function calls into the pico-sdk dynamic timer framework which sets up a
new dynamic timer instance each time, and then has to tear it down before
continuing after a WFE.

Testing Python interrupt latency, it seems to be improved by about 12us
(from average of 46us to 34us running a Pin IRQ).  C-based "scheduled
nodes" should see even lower latency.

This work was funded through GitHub Sponsors.

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

ports/rp2/modmachine.c

@@ -111,7 +111,7 @@ STATIC mp_obj_t machine_freq(size_t n_args, const mp_obj_t *args) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_freq_obj, 0, 1, machine_freq);
 
 STATIC mp_obj_t machine_idle(void) {
- best_effort_wfe_or_timeout(make_timeout_time_ms(1));
+ __wfe();
  return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(machine_idle_obj, machine_idle);

ports/rp2/mpconfigport.h

@@ -274,7 +274,7 @@ extern void mp_thread_end_atomic_section(uint32_t);
 #define MICROPY_EVENT_POLL_HOOK \
  do { \
  MICROPY_EVENT_POLL_HOOK_FAST; \
- best_effort_wfe_or_timeout(make_timeout_time_ms(1)); \
+ __wfe(); \
  } while (0);
 
 #define MICROPY_MAKE_POINTER_CALLABLE(p) ((void *)((mp_uint_t)(p) | 1))