[dma] Use virtual addresses for dma_map()

[dma] Use virtual addresses for dma_map()

Cache management operations must generally be performed on virtual
addresses rather than physical addresses.

Change the address parameter in dma_map() to be a virtual address, and
make dma() the API-level primitive instead of dma_phys().

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