dt-bindings: Clean-up OPP binding node names in examples

In preparation to convert OPP bindings to DT schema, clean-up a few OPP
binding node names in the binding examples.

Cc: Georgi Djakov <djakov@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Georgi Djakov <djakov@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>