[RFC] Add support for multiple CS pin for SDRAM controller

[t123yh]

This patch adds support for multiple CS pin, useful if anyone wants 128MB or 256MB with SDRAM. It's tested on a board with 4 IS42S16320F SDRAM chips, forming 32bit x 2CS structure.

Note that it currently only works with the sdr controller, not xdr controller. I'm not sure how to modify xdr controller, so any help is appreciated.

[Dolu1990]

I guess
https://github.com/SpinalHDL/SpinalHDL/runs/4786705600?check_suite_focus=true#step:10:167
regs.CSn(0) := io.ctrl.phases(0).CSn should fix the compilation error