integration/soc: make DMA slave region cover (at least) the lower 4GB

integration/soc: make DMA slave region cover (at least) the lower 4GB

Assuming we currently support a 32-bit (4GB) physical address space,
ensure that the dma_bus slave covers the entire range, covering any
possible layout of the LiteX SoC memory map (e.g., rocket has MMIO
in a wide range of registers located below 2GB, and DRAM starting at
the 2GB mark, needing DMA accesses to be routed appropriately for the
entire 4GB physical address range).

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>