cpufreq: imx-cpufreq-dt: Add "cpu-supply" property check

The cpufreq-dt driver allows cpufreq driver enabled without valid
regulator assigned, however, all i.MX platforms using cpufreq-dt
driver now require valid regulator, add "cpu-supply" property check
to avoid i.MX platforms' cpufreq enabled without valid regulator
and lead to system unstable.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>