Atom2K18: Add a compatibility mode for slow Devices

[hoglet67]

This mostly concerns external devices, like the external 6522 at #B810

When the Atom CPU is running at 8MHz, the external Phi2 clock is also 8MHz, and devices like the 6522 don't work well.

One answer is just to fit a W65C22N, which is specced to run at 14MHz.

Another answer is to add wait states, and slow down the external phi2.