Documentation: ACPI: move cppc_sysfs.txt to admin-guide/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>