OPP: Add support to find OPP for a set of keys

Some clients, such as PCIe, may operate at the same clock frequency
across different data rates by varying link width. In such cases,
frequency alone is not sufficient to uniquely identify an OPP.

To support these scenarios, introduce a new API
dev_pm_opp_find_key_exact() that allows OPP lookup with different
set of keys like freq, level & bandwidth.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[ Viresh: Minor cleanups ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>