dt-bindings: net: wireless: mt76: document bindings for MT7986

Add an entry for MT7986 SoC.

Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>