Lattice Crosslink-NX EVN support

[josuah]

This issue is there to allow to reproduce more easily the several recent merge requests associated with the support of that board:

• [RFC] remove SD-card support, absent from Zephyr enjoy-digital/litex#1747
• Generate DTS with XIP support for running Zephyr from flash enjoy-digital/litex#1748
• Slow but reliable ecpprog enjoy-digital/litex#1749
• [RFC] More like Linux on Litex VexRiscv #17
• Add support for Crosslink NX EVN #19
• SPI flash support to boot larger firmwares #13
• board: riscv: litex_vexriscv: add flash XIP support zephyrproject-rtos/zephyr#61640

Once everything gets merged on LiteX-side, it would be a good time to remove the [draft] flag off the Zephyr one.

[josuah]

Here is a script to do everything on top of the appropriate LiteX and Zephyr forks:
zephyr_on_litex_on_crosslink_nx_evn.txt

1. modify the hardware as described by LiteX:

   • Remove R18 and R19 and populate R15 and R17
   • Put FTDI UART on port B: FTProg or https://github.com/trabucayre/fixFT2232_ecp5evn

2. Run all script steps:
   sh zephyr_on_litex_on_crosslink_nx_evn.txt all

3. Connect to the LiteX shell

   litex_term /dev/ttyUSB1
   LiteX> boot 0x01000000
   uart$
   

[josuah]

All the submissions that are left are RFCs, the other ones were not essential.
Nothing left here! The remaining [RFC]s can live on their own.
Thank you for watching! :)