[riscv] Use 1GB pages for I/O device mappings

[riscv] Use 1GB pages for I/O device mappings

All 64-bit paging schemes support at least 1GB "gigapages".  Use these
to map I/O devices instead of 2MB "megapages".  This reduces the
number of consumed page table entries, increases the visual similarity
of I/O remapped addresses to the underlying physical addresses, and
opens up the possibility of reusing the code to create the coherent
DMA map of the 32-bit address space.

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