riscv: add DMA mappings suitable for a pure 64-bit system

All RISC-V platforms today lack an IOMMU. However, legacy PCI devices sometimes
require DMA-memory to be in the low 32 bits. To make this work, we enable the
software-based bounce buffers from swiotlb. They only impose overhead when the
device in question cannot address the full 64-bit address space, so a perfect fit.

This patch assumes that DMA is coherent with the processor and the PCI bus. It
also assumes that the processor and devices share a common address space. This is
true for all RISC-V platforms so far.