opp: Manage empty OPP tables with clk handle

With OPP core now supporting DVFS for IO devices, we have instances of
IO devices (same IP block) which require an OPP on some platforms/SoCs
while just needing to scale the clock on some others.

In order to avoid conditional code in every driver which supports such
devices (to check for availability of OPPs and then deciding to do
either dev_pm_opp_set_rate() or clk_set_rate()) add support to manage
empty OPP tables with a clk handle.

This makes dev_pm_opp_set_rate() equivalent of a clk_set_rate() for
devices with just a clk and no OPPs specified, and makes
dev_pm_opp_set_rate(0) bail out without throwing an error.

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>