Experiment on how to implement low power mode on Noridc52840

主要实现Nordic Sleep mode，并且让外界中断可以唤醒Nordic的休眠模式

1. be able enter low power / sleep mode
2. to be able to demonstrate it enters low power mode
3. to be able to be waken up by external interrupt

Question to think during implementation:

• How can you prove that Nordic enters sleep mode successfully?
• How to put Nordic into low power mode that can be woken up by the external interrupt?
• How to electrically connect External interrupt source to Nordic to do this waking up?
• will you set up some sort of push button simulated pin raise/lower to test wake up feature?
• will you set up some sort of power metering to detect power usage?

• this code is built and tested on segger embedded studio 4.50
• this code is built and tested with Nordic's nRF5_SDK_15.2.0
• this code is built and tested with either Nordic52840 or NINAB301 DK
• if any experience/knowldege is learnt while developing this task, this notice will be updated.

[S] this is a reference Nordic project for a starting point for low power implmentation nRF5_SDK_15.2.0\examples\peripheral\pwr_mgmt

[S] To try to use these functions to achieve low power: they are being used by previous Boost Design projects (fw_smartaed_button) on low power mode: power_management_init(); idle_state_handle(); nrf_pwr_mgmt_run(); sleep_mode_enter();