dfu: nand: Add missing dependency on CMD_MTDPARTS

dfu: nand: Add missing dependency on CMD_MTDPARTS

dfu_fill_entity_nand() uses find_dev_and_part() and mtdparts_init()
which are provided by cmd/mtdparts.c.

Add the dependency to avoid build failures when CMD_MTDPARTS is not
selected.

Reported-by: Jagan Teki <jagan@amarulasolutions.com>
Fixes: 6828e602b722d ("dfu: Migrate to Kconfig")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>