cpufreq: scpi: invoke frequency-invariance setter function

Commit 343a8d17 (cpufreq: scpi: remove arm_big_little dependency)
changed the cpufreq driver on juno from arm_big_little to scpi.

The scpi set_target function does not call the frequency-invariance
setter function arch_set_freq_scale() like the arm_big_little set_target
function does. As a result the task scheduler load and utilization
signals are not frequency-invariant on this platform anymore.

Fix this by adding a call to arch_set_freq_scale() into
scpi_cpufreq_set_target().

Fixes: 343a8d17 (cpufreq: scpi: remove arm_big_little dependency)
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>