mtd: rawnand: Prepare things to reuse the generic NAND layer

The generic NAND layer provides abstraction of NAND devices no matter
the bus that is used to communicate with the chip. Basing the raw NAND
core on this generic layer should avoid duplication of common
operations, like iterating over all pages/blocks for MTD IO/erase
operations.

In order to re-use this layer, we must first inherit from nand_device
and then initialize the nand_device struct appropriately. This patch
is taking care of the former.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>