ARM: rockchip: honor renaming of cpufreq-cpu0 to cpufreq-dt

cpufreq-cpu0 got renamed to a more generic name cpufreq-dt, so
adapt the created platform-device to this.

Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>