mtd: plat-nand: drop unused fields from platform_nand_data

Drop now unused set_parts from struct platform_nand_data. Also, while we are
at it, drop long unused priv field from platform_nand_data.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>