mtd: add init_size hook for NAND driver

Not all the NAND devices have all the information in additional
id bytes.

So add a hook in the nand_chip{} is a good method to calculate the
right value of oobsize, erasesize and so on.

Without the hook,you will get the wrong value, and you have to hack
in the ->scan_bbt() to change the wrong value which make the code
mess.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>