docs: Add PECI documentation

Add a brief overview of PECI and PECI wire interface.
The documentation also contains kernel-doc for PECI subsystem internals
and PECI CPU Driver API.

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Iwona Winiarska <iwona.winiarska@intel.com>
Link: https://lore.kernel.org/r/20220208153639.255278-14-iwona.winiarska@intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>