mtd: nand: add lock/unlock routines

Add nand lock / unlock routines. At least 'micron' parts
support this.

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>