[Coco] Instruction cycles per 60Hz frame.

[Alex Evans]

Ok, even simpler (considering that the CoCo does not normally send a n
interlaced signal) According to some m6847 specs (so we are talking
CoCo2 for the moment. A line takes 228 color burst cycles (the 6847
datasheet says 227.5, but testing seems to say 228) and a frame 262
lines. This gives us exactly 14934 CPU cycles per line on a CoCo 2 at
.89 MHz. If we presume that timing is the same for teh CoCo3, then it
would be 29868 CPU cycles at 1.78 MHz.

https://github.com/mamedev/mame/blob/master/src/devices/video/mc6847.cpp