[Task] Provide an Arduino implementation to disable several channels.

[ouphi]

Story: #XX

Description
To work with the CN470 and AU915 Arduino samples, we need to disable channels.
For that, we add the custom setChannelOFF method to the LoRaWan cpp library.
In the Arduino samples for CN470 and AU915 we call setChannelOFF in a for loop to disable one by one each channel within a range.

It seems that there is an AT command that can disable several channels within a range, in a faster way.
AT+CH=NUM, <start>-<end>, OFF
We could use this implementation instead, to disable the channels faster.

Acceptance criteria

• The instructions to add the method implementation is documented in the Arduino Samples doc
• The new method is used in the Arduino samples for CN470 and AU915 regions

[Mandur]

This will be removed from the Hackoverflow project and tackled later