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

  ddrc-opp-table: $nodename:0: 'ddrc-opp-table' does not match '^opp-table(-[a-z0-9]+)?$'
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>