Regression - Simulation boot ram offset for rv64

[troibe]

For https://github.com/enjoy-digital/litex/pull/1259 I just tried running lxsim --cpu-type blackparrot --cpu-variant standard --with-sdram --sdram-init demo.bin.

Unfortunately it seems like the boot ram offset is now fixed to 0x40000000 used by most rv32 CPUs here: https://github.com/enjoy-digital/litex/blob/c400479174847a3f107236badc156d9dbd8fd41e/litex/tools/litex_sim.py#L411

I remember I submitted a similar patch last year but the code seems a bit different this year.

[enjoy-digital]

Thanks @developandplay for reporting, I'll have a look at it. The initialization code has been changed to add .json support and as you saw this is a remaining point to take into account.