arm64: dts: mediatek: update mt7986a memory node

This patch updates the format of memory node in DT adding the correct
device_type.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Link: https://lore.kernel.org/r/20220119123537.9968-2-sam.shih@mediatek.com


[mb: fixed commit message]
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>