Implement 40-bit debug timer (Timer4)

[daniel5151]

Timer 4 is the 40 bit debug timer (as described under section 18.1.3). It's similar to Timers 1-3, albeit without as much "customization" options.

It should be fairly easy to implement:

• Modify the current timer device to use a internal u64 counter for time-keeping
• Create a "wrapper" device which masks masks out any timer registers unavailable on Timer 4
• Update the Ts7200Bus structure + memory map with the new device