There are many options on the way from Porto, the infrastructure is excellent there. I am a tea drinker myself, so I usually take teabags with me when I travel, green tea with jasmin and some herbal as well. Portugal has no tea culture, but you could find green and black tea if you ask for it in the cafes. Portugal is all about coffee though, I usually had my first espresso after a couple of hours of walking, of course with pastel de nata
Most of the albergues in Portugal have kitchens, so you can cook or make tea in the morning. In Galicia it's another story though as the municipal ones are bare, no kitchen equipments there whatsoever, even a kettle. So there be flexible and get it in the cafes on the way or stay in the private one where you could use the amenities. You may find it peculiar to see how your dependance on morning coffee routine might change on the way, but when you get back home you most probably feel that coffe there is just bearable in comparison