cpufreq: dt: Add support for zx296718

Add the compatible string for supporting the generic cpufreq driver on
the ZTE's zx296718 SoC.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>