OPP: add index check to assert to avoid buffer overflow in _read_freq()

Pass the freq index to the assert function to make sure
we do not read a freq out of the opp->rates[] table when called
from the indexed variants:
dev_pm_opp_find_freq_exact_indexed() or
dev_pm_opp_find_freq_ceil/floor_indexed().

Add a secondary parameter to the assert function, unused
for assert_single_clk() then add assert_clk_index() which
will check for the clock index when called from the _indexed()
find functions.

Fixes: 142e17c1 ("OPP: Introduce dev_pm_opp_find_freq_{ceil/floor}_indexed() APIs")
Fixes: a5893928 ("OPP: Add dev_pm_opp_find_freq_exact_indexed()")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>