mtd: nand: davinci: move bindings under mtd

Move bindings under mtd. Do this in order to make davinci-nand
driver usable by keystone architecture.
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Taras Kondratiuk <taras@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>