dt-bindings: arm: mediatek: update for MT7623n SoC and relevant boards

Because there are two versions of MT7623 SoC that is MT7623a and MT7623n
respectively. So update the part of MT7623n bindings to allow that people
tend to differentiate which MT7623 SoC the boards applies.

And "mediatek,mt7623-evb" can be safely changed to
"mediatek,mt7623n-rfb-nand" because mt7623-evb is a kind of debug board
internally in Mediatek which real users can't get. So instead we should
indicate which variants it belongs to with more specific postfix as the
adding here to let people easily know what board they use.

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>