Trigger In/Out

[AleksaBjelogrlic]

Starting this issue to centralize technical discussions on this here - moving away from using the discord for those.

This should wait till Rev 5, since the LVDS used for this is synced with the ADC and can get us sample by sample external trigger states.

[NateMeyer]

Added gateware support for External Sync control EEVengers/thunderscope_litex@cc8a758

[AleksaBjelogrlic]

I'll keep this issue open for trigger out. The above should give us trigger in

[NateMeyer]

Gateware support for trigger out is included. Software support is still needed.