video: fbdev: imxfb: Drop platform data support

No source file but the driver itself includes the header containing the
platform data definition. The last user is gone since commit
8485adf1 ("ARM: imx: Remove imx device directory").

So we can safely drop platform data support.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Helge Deller <deller@gmx.de>