opp: core: implement dev_pm_opp_get_bw

Add and implement dev_pm_opp_get_bw() to retrieve the OPP's
bandwidth in the same way as the dev_pm_opp_get_voltage() helper.

Retrieving bandwidth is required in the case of the Adreno GPU
where the GPU Management Unit can handle the Bandwidth scaling.

The helper can get the peak or average bandwidth for any of
the interconnect path.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
[ Viresh: Fixed commit log and a comment in code ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>