PM / OPP: Add reference counting helpers for Rust implementation

To ensure that resources such as OPP tables or OPP nodes are not freed
while in use by the Rust implementation, it is necessary to increment
their reference count from Rust code.

This commit introduces a new helper function,
dev_pm_opp_get_opp_table_ref(), to increment the reference count of an
OPP table and declares the existing helper dev_pm_opp_get() in pm_opp.h.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>