cpufreq: qcom-cpufreq-hw: use HZ_PER_KHZ macro in units.h

HZ macros has been centralized in units.h since [1]. Use it to avoid
duplicated definition.

[1] commit e2c77032 ("units: add the HZ macros")

Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>