opp: Fix required-opps phandle array count check

The 'required-opps' property is optional. So of_count_phandle_with_args()
can return -ENOENT when queried for required-opps. Handle this case.

Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>