For the charging iPhone6+, yes the AC adapter is 12V 2A and total is 24W so it looks it is not problem for charging iPhone and iPad which draw the current in fastmode from 1A (5W) to 2.4A (12W).
However, the 5V line on USB connector is made by DC-DC converter on the HiKey from 12V and this 12V DC- 5V DC converter is limited to maximum 2.6A. Furthermore, 3.3V is made from this limited 5V and all the components using 5V and 3.3V on the Hikey are consuming the same 5V 2.5A power.
Regular USB charging limit is 5V 0.5A which is 2.5W, so this would not be the problem.
There are USB charger cable for iPhone and that the USB connector has D- pin with 2.7V which indicate the iPhone to use fastmode charging on regular USB connector as described bellow.
But if you try to charge iPhone with fastmode which is Apple’s extension, will draws 1A (5W) to 2.4A (12W),
then it could overload the DC-DC converter on the HiKey, since USB specification only specifies 0.5A on the USB connector,
and the HiKey’s design is completely complaint only on the USB specification and do not look like supporting fastmode on Apple.
It is safer not to charge iPhone and iPod on regular USB connector.