Suggested-by: @kjbracey-arm
Replace the sleep_manager_can_deep_sleep() with !_deep_sleep_locked.
Indeed, if we know we've taken the lock because we're using us_ticker,
no need to do the early wake.
Updated comments accordingly.
When next SysTimer wake-up is scheduler far enough, always consider
that deep sleep may be entered and program an early wake-up.
So that even if deep sleep is only allowed some time later, it can be
entered. If not doing this, then the deep sleep would be prevented by
SysTimer itself and may not be entered at all.
This has been proved to happen in a simple blinly example.
Mika Leppänen
Laurent Meunier
Hugues Kamba
George Psimenos