cpufreq: dt: Add rk3328 compatible to use generic cpufreq driver

This patch adds the rk3328 compatible string for supporting
the generic cpufreq driver on RK3328.

Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>