cpufreq: scmi: Set transition_delay_us

Make use of the newly added callbacks:
- rate_limit_get()
- fast_switch_rate_limit()
to populate policies's `transition_delay_us`, defined as the
'Preferred average time interval between consecutive
invocations of the driver to set the frequency for this policy.'

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>