tools/power x86_energy_perf_policy.8: Emphasize preference for SW interfaces

tools/power x86_energy_perf_policy.8: Emphasize preference for SW interfaces

This tool was originally written when Linux had no standard
interface for EPB, or HWP support.

Retain the capability to manage a system w/o any kernel PM support,
but prefer the standard kernel interfaces, when avaialble.

(not doing so led to a confusing conflict between a p-state limit
 request made via cpufreq and modified by the intel-pstate driver,
 versus the raw MSR write made by this utility)

Signed-off-by: Len Brown <len.brown@intel.com>