mtd: nand: mediatek: update DT bindings

Add MT2712 NAND Flash Controller dt bindings documentation.

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>