I hear they have been designing their own GPU for many years now, and I'm quite (half)surprised they haven't released anything yet: one does not simply acquire the 30-year expertise of Imagination in such a short timespan.
NVIDIA tried going in the mobile SoC business, and after 5 generations and billions burned they finally gave up. What Apple did (and keeps doing, by putting Samsung, Qualcomm to shame[1]), is mind boggling. They are even approaching Intel's perf-per-watt[2] ratio.
[1] http://anandtech.com/show/9686/the-apple-iphone-6s-and-iphon...
[2]: http://anandtech.com/show/9766/the-apple-ipad-pro-review/4