"Loco" is the Spanish word meaning "crazy", "mad", etc, and, as with those adjectives, when followed by "for" or "about" can form a phrase meaning "very fond of, or enthusiastic about". "Loco for coco" can be a short and casual way of saying "mad about chocolate or coconut". "Loco for Coco" is the name of a company that sells chocolate. It probably came about because people creating brand names often want them to be short, and to rhyme. Coco is the name of an animated movie, and of a character in that movie, but these are nothing to do with coconuts or chocolate.