docs: security: Add secrets/coco documentation

Add documentation for the efi_secret module which allows access
to Confidential Computing injected secrets.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://lore.kernel.org/r/20220412212127.154182-5-dovmurik@linux.ibm.com


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>