cpufreq: imx-cpufreq-dt: Remove global platform match list

This is not currently needed, instead a platform device is always created
from SOC-specific code.

We can use of_machine_is_compatible for per-SOC behavior instead.

Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>