Implement fans about mk404

[vintagepc]

Fans implemented and attached, but not entirely functional yet.
Blocked by #30 as the timer4 PWM does not start by default.

Values can be set and show when used with fan_set_rpm().

Also note... the PWM control might be swapped (wasn't clear which fan was which in the pin definitions) and the fan on the PWM2 IRQ might not work at all yet as the core simAVR does not emit this signal.

[vintagepc]

Done. No idea how well the print fan will behave though as it is soft-PWMed and doesn't have any wind-down speed like the real fan does.

I believe the reason it is done like this has to do with reading the tach value and also the low-frequency whine of the default PWM timer.

[vintagepc]

Print fan reads back just fine using soft PWM