mtd: pxa3xx_nand: Remove unused platform-data flash specification

The driver supports board files specificating the flash
device, by passing a pxa3xx_nand_flash struct (with
flash parameters) in the platform data struct.

Currently this support is not being used by any board file.
Moreover, we'd like to deprecate such usage in favor of
using the device table in nand_ids.c.

So let's remove the ad-hoc flash specification.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>