OPP: Fix parsing of multiple phandles in "operating-points-v2" property

We currently return error if more than one phandle is present in the
"operating-points-v2" property, which is incorrect. We only want to
check the count of phandles here and set index to 0 if only one phandle
is present.

Fix it.

Fixes: 5ed4cecd ("OPP: Pass OPP table to _of_add_opp_table_v{1|2}()")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>