arm64: dts: mediatek: mt7622: drop serial clock-names

The serial node does not use clock-names:

  mt7622-bananapi-bpi-r64.dtb: serial@1100c000: Unevaluated properties are not allowed ('clock-names' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230123151634.370011-1-krzysztof.kozlowski@linaro.org


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