mtd: rawnand: mxc: Remove platform data support

i.MX is a devicetree-only platform now and the existing platform data
support in this driver was only useful for old non-devicetree platforms.

Get rid of the platform data support since it is no longer used.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201110121908.19400-1-festevam@gmail.com