cpu/vexriscv_smp: more coherent_dma to __init__ instead of add_memory_buses.

cpu/vexriscv_smp: more coherent_dma to __init__ instead of add_memory_buses.

LiteX is creating the SoC.dma_bus just after the CPU is declared, so declaring it in add_memory_buses was preventing it.
It's also more coherent to move it to __init__ since not related to the memory_buses.