[riscv] Create coherent DMA mapping of 32-bit address space on demand

[riscv] Create coherent DMA mapping of 32-bit address space on demand

Reuse the code that creates I/O device page mappings to create the
coherent DMA mapping of the 32-bit address space on demand, instead of
constructing this mapping as part of the initial page table.

Signed-off-by: Michael Brown <mcb30@ipxe.org>