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.

[sy2002]

@oe3gwu Nice feature request. Thank you. The underlying MiSTer core does support several turbo modes, see

https://github.com/MJoergen/C64_MiSTerMEGA65/blob/1c6bd485e7d71d4c89348a82ae875de739d7e267/c64.sv#L261

and

https://github.com/MJoergen/C64_MiSTerMEGA65/blob/1c6bd485e7d71d4c89348a82ae875de739d7e267/c64.sv#L937

so @MJoergen when / as soon as we tackle this one we might want to research how these work.

[MJoergen]

Remember to take into account #2 so we avoid any regression bugs