cpufreq: ti-cpufreq: Add support for multiple regulators

Some platforms, like those in the DRA7 and AM57 families, require the
scaling of multiple regulators in order to properly support higher OPPs.
Let the ti-cpufreq driver determine when this is required and pass the
appropriate regulator names to the OPP core so that they can be properly
managed.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>