mtd: Move fdt partition documentation to a seperate file

Partitions are described in the same way for all mtd devices when using
devicetree, move the documentation to a separate file and add references
to it.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>