Align the name of operating-points node to dtschema to fix warnings like:

  arch/arm64/boot/dts/mediatek/mt8173-elm.dt.yaml:
    opp_table0: $nodename:0: 'opp_table0' does not match '^opp-table(-[a-z0-9]+)?$'
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20210820081616.83674-1-krzysztof.kozlowski@canonical.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>