OPP: Add helpers for reading the binding properties

OPP: Add helpers for reading the binding properties

The opp-hz DT property is not mandatory and we may use another property
as a key in the OPP table. Add helper functions to simplify the reading
and comparing the keys.

Signed-off-by: Saravana Kannan <[email protected]>
Signed-off-by: Georgi Djakov <[email protected]>
Reviewed-by: Matthias Kaehlcke <[email protected]>
Reviewed-by: Sibi Sankar <[email protected]>
[ Viresh: Removed an unnecessary comment ]
Signed-off-by: Viresh Kumar <[email protected]>