arm64: dts: zx: support cpu-freq for zx296718

This patch adds the CPU clock phandle in CPU's node
and uses operating-points-v2 to register operating points.

So it can be used by cpufreq-dt driver.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>