Add Example Code for Multiple Channel Scan Mode SAADC of nrf52

[shanyechungu]

The example code for the SAADC of nrf52 is a single channel one-shot mode which is not very helpful in practical use. For most of the projects, people need more than just one channel for their ADC tasks. In the ADC example or the API provided, there is less information on how to configure the ADC to Scan mode and retrive the results from multiple channels.

Please provide an example code for at least two channels in scan mode.

Thanks,