Add an option to change the Processor Clock Frequency

[oe3gwu]

It would be nice to have the same clock options as on the MEGA65. For example 1, 2, 4 , ... 40 MHz.

The reason is that Test Drive II is utterly really slow using 1MHz. And it is kinda playable on a C128 using 2MHz and it rocks using 4MHz using my Ultimate 64.