mtd: nandsim: Embed struct nand_chip in struct nandsim

We will need struct nand_controller soon, so more stuff need to
be parts of struct nandsim.
While we are here, rename "nand" to "ns" to use the same naming scheme
everywhere in nandsim.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>