dt-bindings: opp: Introduce opp-peak-kBps and opp-avg-kBps bindings

Interconnects often quantify their performance points in terms of
bandwidth. So, add opp-peak-kBps (required) and opp-avg-kBps (optional) to
allow specifying Bandwidth OPP tables in DT.

opp-peak-kBps is a required property that replaces opp-hz for Bandwidth OPP
tables.

opp-avg-kBps is an optional property that can be used in Bandwidth OPP
tables.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Reviewed-by: Sibi Sankar <sibis@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>