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>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 280496f..18dbc23 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -487,7 +487,7 @@
 	help
 	  Enable support for NAND (Negative AND) flash in SPL. NAND flash
 	  can be used to allow SPL to load U-Boot from supported devices.
-	  This enables the drivers in drivers/mtd/nand as part of an SPL
+	  This enables the drivers in drivers/mtd/nand/raw as part of an SPL
 	  build.
 
 config SPL_NET_SUPPORT