Hi, Anne,
Very sorry to tell you that unless things have changed dramatically, there are virtually no laundromats on the Camino del Norte. For whatever reason, self-service laundromats are just not common in Spain. What you may find (for example in Santiago there's one and also in Santo Domingo de la Calzada) are lavanderias where you bring your clothes and drop them off, and someone else washes and folds them.
BUT I think you will find a growing number of albergues that have washing machines and occasionally dryers. I was surprised to find that even on the very untraveled Camino del Levante there were washers in many albergues. Usually the service costs 3-5 euros. Also, if you ever stay in a Casa Rural or often in small pensiones, you can get the owners to do a load of wash for you at a very reasonable price, or even for free.
Buen camino, Laurie