cpufreq: dt: Pass regulator name to the OPP core

OPP core can handle the regulators by itself, and but it needs to know
the name of the regulator to fetch. Add support for that.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 050794aaebbb9f2c2c50b340b6998273e7c64189)
Signed-off-by: Alex Shi <alex.shi@linaro.org>