mtd: move NAND files into a raw/ subdirectory

NAND flavors, like serial and parallel, have a lot in common and would
benefit to share code. Let's move raw (parallel) NAND specific code in a
raw/ subdirectory, to ease the addition of a core file in nand/ and the
introduction of a spi/ subdirectory specific to SPI NANDs.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>