arm: dts: mt7623: cleanup MT7623N NAND dts file

Normally, we didn't release this kind of baord to user. This specific
board exists only in the early stage of development inside MediaTek -
and that may confuse peoples.

Hence this patch removes related files accordingly.

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