firmware: arm_scmi: Add fast_switch_possible() interface

Add a new fast_switch_possible interface to the existing perf_ops to
export the information of whether or not fast_switch is possible for a
given device.

This can be used by the cpufreq driver and framework to choose proper
mechanism for frequency change.

Link: https://lore.kernel.org/r/20200617094332.8391-1-nicola.mazzucato@arm.com


Suggested-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>