opp: Add sanity checks in _read_opp_key()

When we read the OPP keys, it would be nice to do some sanity checks
of the values we get from DT and see if they match with the information
that is populated in the OPP table. Let's pass a pointer of the table,
so that we can do some validation.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Sibi Sankar <sibis@codeaurora.org>
[ Viresh: Fix rebase conflicts ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>