esp8266: Implement basic deep-sleep capabilities.

Use the machine.deepsleep() function to enter the sleep mode.  Use the
RTC to configure the alarm to wake the device.

Basic use is the following:

    import machine

    # configure RTC's ALARM0 to wake device from deep sleep
    rtc = machine.RTC()
    rtc.irq(trigger=rtc.ALARM0, wake=machine.DEEPSLEEP)

    # do other things
    # ...

    # set ALARM0's alarm to wake after 10 seconds
    rtc.alarm(rtc.ALARM0, 10000)

    # enter deep-sleep state (system is reset upon waking)
    machine.deepsleep()

To detect if the system woke from a deep sleep use:

    if machine.reset_cause() == machine.DEEPSLEEP_RESET:
        print('woke from deep sleep')