mtd: rawnand: toshiba: Support reading the number of bitflips for BENAND (Built-in ECC NAND)

Add support vendor specific commands for KIOXIA CORPORATION BENAND.
The actual bitflips number can be retrieved by this command.

Signed-off-by: Yoshio Furuyama <ytc-mb-yfuruyama7@kioxia.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1585124572-4693-1-git-send-email-ytc-mb-yfuruyama7@kioxia.com