soc_linux: use new max_sdram_size paramter of SoCSDRAM to limit mapped SDRAM to 256MB
soc_linux: use new max_sdram_size paramter of SoCSDRAM to limit mapped SDRAM to 256MB

litex-hub/linux-on-litex-vexriscv,litex-hub/linux-on-litex-vexriscv