cpufreq: arm_scmi: Set fast_switch_possible conditionally

Currently the fast_switch_possible flag is set unconditionally to true.
Based on this, schedutil does not create a thread for frequency
switching and would always use the fast switch path.

However, if the platform does not support SCMI fast channel, we use
polling mode for SCMI message transfer. This may be possible only if
there is dedicated channel for DVFS and all operations are in polling
mode.

Update this by retrieving the fast_switch capability based on the
presence of fast channels in SCMI platform firmware.

Link: https://lore.kernel.org/r/20200617094332.8391-2-nicola.mazzucato@arm.com


Suggested-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>