tools/power/x86/intel-speed-select: Make process_command generic

Make the process_command take any help command and command list.  This
will make it easier to help commands and a command list for CascadeLake-N.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>