arm64: mediatek: cleanup message for platform selection

The latest kernel tree already can support more MediaTek platforms such as
MT2712 and MT7622, so additional descriptions for those platforms are added
and certain cleanups are also being made here.

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