Add support for CAN in litex

[disdi]

This is to add support for controller area network (CAN bus) to Litex.
It is based on CTU CAN FD IP Core developed by Czech Technical University in Prague.

It also uses work done by Antmicro wrapper.

Complete documentation of usage and testing can be found at Project Wiki.

Old MR: #1871

[enjoy-digital]

Thanks @disdi and sorry for the delay. We also integrated this core a few years ago as a collaboration with @AndrewD (code here: https://github.com/AndrewD/litecan) and the way the core is integrated better matches the approach we want to push for LiteX integrated core. We are currently reworking things with @trabucayre to be able to easily reuse the core and you could also switch to it when done. The .dts part of your PR should however be the same and we could probably merge it when we'll be done with this rework. Thanks a lot for your contribution and sorry to not being able to merge as is.

[disdi]

@enjoy-digital sounds good to me as long as it gets CAN bus support in Litex

[enjoy-digital]

@disdi: The work I was describing in #1900 (comment) has been integrated through #2007. Sorry for not merging as is and thanks for your contribution! I'm going to merge litex-hub/litex-boards#570 and adapt it to the new integration.