mtd: ensure MTD/the raw NAND core are compiled when there is a NAND flash

Both symbols must be enabled when there is a raw NAND driver
selected. Also enable them when CONFIG_CMD_NAND is selected to avoid
breaking things when we'll further rework the MTD dependency
description.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
[trini: Rebase and migrate a few more boards here]
Signed-off-by: Tom Rini <trini@konsulko.com>