mtd: rawnand: Move platform_nand_xxx definitions out of rawnand.h

platform_nand_xxx definitions are just used by the plat_nand driver.
Let's move those definitions out of the core/driver-agnostic rawnand.h
header.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>