[CPUFREQ][2/8] acpi: reorganize code to make MSR support addition easier

Some clean up and redsign of the driver. Mainly making it easier to add
support for multiple sub-mechanisms of changing frequency. Currently this
driver supports only ACPI SYSTEM_IO address space. With the changes
below it is easier to add support for other address spaces like Intel
Enhanced Speedstep which uses MSR (ACPI FIXED_FEATURE_HARDWARE) to do the
transitions.

Signed-off-by: Denis Sadykov <denis.m.sadykov@intel.com>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Alexey Starikovskiy <alexey.y.starikovskiy@intel.com>
Signed-off-by: Dave Jones <davej@redhat.com>