PM / OPP: dt-bindings: Rename "required-opp" as "required-opps"

This property can contain more than one phandle and it must be named
"required-opps" instead.

Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>