NOMMU: Add support for direct mapping through mtdconcat if possible

Add support for direct mapping through mtdconcat, if possible, by attaching the
samebacking_dev_info structure to the master.

It has some restrictions:

 (1) It won't permit direct mapping of concatenated devices that have differing
     BDIs.

 (2) It doesn't support maps that span the 'gap' between devices, although it
     possibly could if the devices spanned across return compatible
     (ie. contiguous) addresses from their get_unmapped_area() ops.

Signed-off-by: Gavin Lambert <gavinl@compacsort.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>