mtd: rawnand: omap2: Use nand_controller_init()

Stop initializing omap_gpmc_controller fields are declaration time and
replace that by a call to nand_controller_init(). Since the same object
might be shared by several NAND chips and the NAND controller driver
expects a ->probe() per-chip, we need to keep track of the
omap_gpmc_controller state (whether it's already been initialized or
not).

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