mtd: nand: State when references to other drivers are no longer valid

A lot of NAND drivers have been derived from other old NAND drivers
which have since then been removed from the Linux tree. When this is
the case, specify when the file the header is referring to has been
removed so that people can find the original implementation more
easily.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>