mtd: nand: gpmi: add raw oob access functions

Implement raw OOB access functions to retrieve OOB bytes when accessing the
NAND in raw mode.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>