mtd: nand: Stop using full path when referring to files placed in the same dir

Some NAND drivers are derived from other NAND drivers and state it in
their license header. Using full path to point to other driver files
sitting in the same directory is not such a good idea, since the NAND
drivers might be moved to a different directory at some point, and we
don't to patch all license/copyright headers everytime this happen.

The only exception where we keep full path is when the referred source
files no longer exist in the tree.

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