arm: dts: mt7623: enable the nand device on the mt7623n nand rfb

Enable the nand device and setup pinmux on the mt7632m rfb with nand
support.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>