cpufreq: cpu0: try regulators with name "cpu-supply"

Currently, we expect regulator name to be "cpu0", but as we are going to support
multiple cpu-blocks (all CPUs in a block share clock/voltage) later, we need to
pass some generic string instead of that.

For backwards compatibility try for "cpu0" first and if it fails, then try for
"cpu".

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>