mtd: rawnand: rockchip: fix oobfree offset and description

Rockchip boot blocks are written per 4 x 512 byte sectors per page.
Each page with boot blocks must have a page address (PA) pointer in OOB
to the next page.

The currently advertised free OOB area starts at offset 6, like
if 4 PA bytes were located right after the BBM. This is wrong as the
PA bytes are located right before the ECC bytes.

Fix the layout by allowing access to all bytes between the BBM and the
PA bytes instead of reserving 4 bytes right after the BBM.

This change breaks existing jffs2 users.

Fixes: 058e0e84 ("mtd: rawnand: rockchip: NFC driver for RK3308, RK2928 and others")
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/d202f12d-188c-20e8-f2c2-9cc874ad4d22@gmail.com