Having a bike stolen from a campsite is highly unlikely in Norway. If the campsite is close to a city then maybe lock it to something (bench, tree, sign post) close to your tent. Most of the time it will be fine to leave by your tent.

I usually take things of value into my tent at night and if it’s going to rain leave the bags in the outer section of my tent.

Bottomline – Norway is very safe and just a bit of common sense is all you need when camping.