cpufreq: qcom-cpufreq-hw: Fix throttle frequency value on EPSS platforms

On QCOM platforms with EPSS flavour of cpufreq IP a throttled frequency is
obtained from another register REG_DOMAIN_STATE, thus the helper function
qcom_lmh_get_throttle_freq() should be modified accordingly, as for now
it returns gibberish since .reg_current_vote is unset for EPSS hardware.

To exclude a hardcoded magic number 19200 it is replaced by "xo" clock rate
in KHz.

Fixes: 275157b3 ("cpufreq: qcom-cpufreq-hw: Add dcvs interrupt support")
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>