Signal NVM change in multicast management functions

[janakj]

The library provides a number of multicast management functions: LoRaMacMcChannelSetup, LoRaMacMcChannelDelete, LoRaMacMcChannelGetGroupId, and LoRaMacMcChannelSetupRxParams. Some of the functions manipulate data stored in the NVM. Thus, these functions should signal to the NVM layer when the data may have changed so that the NVM can be updated.