It's unlikely. Apple has shown a clear trend towards trying to get rid of absolutely any 3rd party components from their devices. They want the control that full vertical integration gives them.
The got rid of the 3rd party ARM design in their iphones/ipads, going for their own ARM/AARCH64 design. They are currently in the process of getting rid of the PowerVR GPU, to be replaced with their own GPU.
Moving to their own AARCH64 CPU for laptops/desktops fits with that trend. Many people have been wondering 'when' this will happen, not if.
Moving to AMD doesn't fit with this long term trend.
