I don't recall seeing an iron in any of the hotel rooms we stayed at while visiting Spain, but I think the hotel would have an iron, if you ask at the front desk.
Perhaps the high end hotels may have irons in each room, but we stayed in 2-3 star boutique hotels mostly.
Have a great trip!
Edit to add...
Oops! I just realized that I misread the title of your post. Somehow, I'd read it as; do they have clothing irons in the hotel rooms in Spain.
I'm not aware of this rule in Spain, but I didn't use an iron during our trip, so can't really answer your question, my apologies.
You may want to call the hotels you have booked & I'm sure they will give you the answer you're looking for.
Hope you enjoy your trip!