mtd: nand: mtk: update DT bindings

Add MT7622 NAND Flash Controller dt bindings documentation.

Signed-off-by: RogerCC Lin <rogercc.lin@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>