risc-v/litex: For vexriscv_smp, match default FDT address set by upstream opensbi.

[jerenkrantz]

Summary

For vexriscv_smp cores with FDT support, match the upstream opensbi definitions where the FDT should be mapped to.

The default opensbi fw_jump.bin expects to see the FDT in 0x40EF0000 per FW_JUMP_FDT_ADDR in:

0.8 branch: https://github.com/litex-hub/opensbi/blob/a9ce3ada56574fcdb5befae0395f2f3d33134f81/platform/litex/vexriscv/config.mk 1.3.1 branch: https://github.com/litex-hub/opensbi/blob/84c6dc17f7d41c5c02760a5533d7268b57369837/platform/litex/vexriscv/objects.mk

Impact

Without this, the user must override the configuration in either opensbi or in NuttX to match.

Testing

I tested with the arty_a7:knsh config.

[acassis]

@g2gps could you please take a look, since it was you who defined the original memory position: https://github.com/apache/nuttx/pull/11939