mtd: nand: add the "bits per cell" info for legacy ID NAND

The legacy ID NAND are all SLC.
This patch sets 1 to the @bits_per_cell for the legacy ID NAND,
which means they are all SLC.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>