[dma] Define a DMA API to allow for non-flat device address spaces

[dma] Define a DMA API to allow for non-flat device address spaces

iPXE currently assumes that DMA-capable devices can directly address
physical memory using host addresses.  This assumption fails when
using an IOMMU.

Define an internal DMA API with two implementations: a "flat"
implementation for use in legacy BIOS or other environments in which
flat physical addressing is guaranteed to be used and all allocated
physical addresses are guaranteed to be within a 32-bit address space,
and an "operations-based" implementation for use in UEFI or other
environments in which DMA mapping may require bus-specific handling.

The purpose of the fully inlined "flat" implementation is to allow the
trivial identity DMA mappings to be optimised out at build time,
thereby avoiding an increase in code size for legacy BIOS builds.

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