trace: cpu_freq_switch: Add profiler for CPU frequency switch times

It is sometimes useful to profile how long CPU frequency switches
take, and traces have already been added for this purpose. Make
use of these and the trace_stat framework to generate statistical
histograms of frequency switch times in the following format:

 # cat /sys/kernel/debug/tracing/trace_stat/cpu_freq_switch
  CPU START_KHZ  END_KHZ COUNT AVG_US MIN_US MAX_US
    |         |        |     |      |      |      |
    0    384000  1512000     3   2787   1648   3418
    0    486000   384000     1   1129   1129   1129
    0   1458000   384000     1   3174   3174   3174
    0   1512000   384000     1   3265   3265   3265
    0   1512000   486000     1   3235   3235   3235
    0   1512000  1458000     1    213    213    213
    0   1512000  1512000     1      0      0      0

Profiling is disabled by default (since it does incur some
overhead). It can be enabled or re-disabled echoing 1 or 0
to /sys/kernel/debug/tracing/cpu_freq_switch_profile_enabled

Change-Id: I3ef7f9d681b7bd13bcaa031003b10312afe1aefe
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
(cherry picked from commit a629fb0b67b57cc6759da51b9c12750758736c80)
trace: power: add cpu_frequency_switch_{start, end}

It is sometimes useful to profile how long CPU frequency switches
take, since they often involve variable overhead (PLL lock times,
voltage increase time, etc.). Add additional traces to to make this
possible.

Since the overhead involved may differ based on the frequencies
being switched between, record both the start and the end frequencies
as part of the trace.

Change-Id: I2de743fc357dad3590fd4980f65f38f6073d426e
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
(cherry picked from commit 0fe22de09613ce497561f51151b2c50b6c4a1c3e)
(cherry picked from commit 9ab0adbcac4df4d681617a8368329d2f4cad4171)