docs: cpu-freq: convert core.txt to ReST

- Add a SPDX header;
- Adjust the  document title, based on the original contents of
  cpu-freq/index.txt;
- Use lists where needed;
- Comment out the existing text-only index;
- use ``foo`` to mark a literal expression with asterisk;
- Adjust some title marks;
- Add table markups;
- Add notes markups;
- Some whitespace fixes and new line breaks;
- Add it to cpu-freq/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>