arm64: dts: mediatek: mt7986: Add SoC compatible

Missing SoC compatible in the board file causes dt bindings check.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Link: https://lore.kernel.org/r/20221114121653.14739-1-matthias.bgg@kernel.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>