lib/dma-noop: Clarify a comment

The next patch in this series will introduce another set of DMA
operations that map 1:1 with memory. Clarify that dma-noop maps
to physical addresses.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>