PM / devfreq: export functions to allow freeing of pmu counters

Memlat and compute governors are made immutable to avoid governor
switch at run time as the devices using these governors are tightly
coupled with hardware features. These governors allocate pmu counters
to count various events used in the algorithm. These counters are
allocated exclusively for algorithm's use.

This leaves limited counters available for other uses in the system
such as profiling in user space using perf tool. Add and export a
function in devfreq memlat governor to remove immutable flag to allow
governor switch, which in turn can free pmu counters.

Similarly add and export a function in rimps_memlat driver to
free pmu counters.

The intended use of these functions is to be called by test modules
to allow freeing of pmu counters at runtime.

Change-Id: I243a3201368d5a82aeaf820e775cdc1aecdfea07
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>