mtd: Fix dependency for MTD_DOC200x

Fix the following build warning:

warning: (MTD_DOC2000 && MTD_DOC2001 && MTD_DOC2001PLUS) selects MTD_NAND_IDS
which has unmet direct dependencies (MTD && MTD_NAND)

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/devices/Kconfig

@@ -191,6 +191,7 @@ comment "Disk-On-Chip Device Drivers"
 
 config MTD_DOC2000
 	tristate "M-Systems Disk-On-Chip 2000 and Millennium (DEPRECATED)"
+	depends on MTD_NAND
 	select MTD_DOCPROBE
 	select MTD_NAND_IDS
 	---help---
@@ -213,6 +214,7 @@ config MTD_DOC2000
 
 config MTD_DOC2001
 	tristate "M-Systems Disk-On-Chip Millennium-only alternative driver (DEPRECATED)"
+	depends on MTD_NAND
 	select MTD_DOCPROBE
 	select MTD_NAND_IDS
 	---help---
@@ -234,6 +236,7 @@ config MTD_DOC2001
 
 config MTD_DOC2001PLUS
 	tristate "M-Systems Disk-On-Chip Millennium Plus"
+	depends on MTD_NAND
 	select MTD_DOCPROBE
 	select MTD_NAND_IDS
 	---help---