As I understand antitrust law, it is legal for a monopoly in one market to expand into an adjacent market. However, if that monopoly stifles competition in the adjacent market in a way that harms consumers, that monopolist’s acts are illegal.
By virtue of Apple integrating the operating system with hardware, iOS has a defacto 100% monopoly on devices compatible with Apple's Ax series chips (e.g. A9 in iPhone 6s). There's nothing illegal about that.
The market for Ax series devices is now larger than the Intel-compatible PC market was at under consideration during the Microsoft antitrust case.
The question then is whether Apple's policies stifle innovation and harm consumers. I think the answer is "yes".