dt-bindings: arm64: dts: mediatek: add compatible for bananapi r3

Add compatible for mt7986 based BananaPi R3 SBC.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221106085034.12582-9-linux@fw-web.de


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