cpufreq / CPPC: Set platform specific transition_delay_us

cpufreq / CPPC: Set platform specific transition_delay_us

Add support to specify platform specific transition_delay_us instead
of using the transition delay derived from PCC.

With commit 3d41386d556d (cpufreq: CPPC: Use transition_delay_us
depending transition_latency) we are setting transition_delay_us
directly and not applying the LATENCY_MULTIPLIER. Because of that,
on Qualcomm Centriq we can end up with a very high rate of frequency
change requests when using the schedutil governor (default
rate_limit_us=10 compared to an earlier value of 10000).

The PCC subspace describes the rate at which the platform can accept
commands on the CPPC's PCC channel. This includes read and write
command on the PCC channel that can be used for reasons other than
frequency transitions. Moreover the same PCC subspace can be used by
multiple freq domains and deriving transition_delay_us from it as we
do now can be sub-optimal.

Moreover if a platform does not use PCC for desired_perf register then
there is no way to compute the transition latency or the delay_us.

CPPC does not have a standard defined mechanism to get the transition
rate or the latency at the moment.

Given the above limitations, it is simpler to have a platform specific
transition_delay_us and rely on PCC derived value only if a platform
specific value is not available.

Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Cc: 4.14+ <stable@vger.kernel.org> # 4.14+
Fixes: 3d41386d556d (cpufreq: CPPC: Use transition_delay_us depending transition_latency)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>