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>