mphalport.h

Prior to this patch there was a large latency for executing scheduled
callbacks when when Python code is sleeping: at the heart of the
implementation of sleep_ms() is a call to vTaskDelay(1), which always
sleeps for one 100Hz tick, before performing another call to
MICROPY_EVENT_POLL_HOOK.

This patch fixes this issue by using FreeRTOS Task Notifications to signal
the main thread that a new callback is pending.