mtd: core: expose ooblayout information via debugfs

mtd: core: expose ooblayout information via debugfs

Add two new debugfs files which allows to determine the OOB layout
used by a given MTD device. This can be useful to verify the current
layout during driver development without adding extra debug code.
The exposed information also makes it easier to analyze NAND dumps
without the need of crawling out the layout from the driver code.

The content of the new debugfs files is similar to this:

    # cat /sys/kernel/debug/mtd/mtd0/ooblayout_ecc
    0      0   49
    1     65   63
    # cat /sys/kernel/debug/mtd/mtd0/ooblayout_free
    0     49   16

Signed-off-by: Gabor Juhos <j4g8y7@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>