dt-bindings: opp: Support multiple opp-suspend properties

Update opp-suspend property's description to support multiple
opp-suspend properties defined in DT, the OPP with highest opp-hz
and with opp-suspend property present will be used as suspend opp.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>