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

  cpu_opp_table: $nodename:0: 'cpu_opp_table' does not match '^opp-table(-[a-z0-9]+)?$'
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>