mtd: SC520CDP should depend on MTD_CONCAT

Toralf Förster found a compile error when CONFIG_MTD_SC520CDP=y and
CONFIG_MTD_CONCAT=n:

drivers/built-in.o: In function `init_sc520cdp':
sc520cdp.c:(.init.text+0xb4de): undefined reference to `mtd_concat_create'
drivers/built-in.o: In function `cleanup_sc520cdp':
sc520cdp.c:(.exit.text+0x14bc): undefined reference to `mtd_concat_destroy'

This patch fixes it.

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: Josh Boyer <jwboyer@gmail.com>