Support SD cards in DDR50 mode

[machdyne]

It appears that support for 1.8V operation was removed in 2020 but maybe there's interest in resupporting it?

Here is an open-source PMOD that supports 1.8V level translation:

https://github.com/machdyne/microsd

It's been been tested with DDR50 mode by @ZipCPU:

https://github.com/ZipCPU/sdspi

Would it be possible to increase sdcard speeds under linux-on-litex-vexriscv if support for DDR50 were added to LiteX?

[enjoy-digital]

Thanks @machdyne for the feedback. DDR50 has indeed been removed to focus in simplicity and portability of the core in different FPGAs. Adding back support should be possible yes and could indeed improve speed on Linux. Not sure I'll be able to spend much time currently on it, but I would happily review/test PR to add it back.