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 <[e-mail]>
Reviewed-by: Grygorii Strashko <[e-mail]>
Reviewed-by: Taras Kondratiuk <[e-mail]>
Signed-off-by: Ivan Khoronzhuk <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>