efi: add API doc entry for ovmf_debug_log

Document the newly added sysfs ABI for accessing the in-memory debug log
provided by OVMF EFI firmware (when enabled)

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>