mtd: Add sysfs attribute for mtd OOB available size

Expose mtd OOB available size by sysfs file. Then users can get available
OOB size by accessing /sys/class/mtd/mtdX/oobavail.

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>