Documentation: fpga: dfl: Add documentation for DFHv1

Add documentation describing the extensions provided by Version
1 of the Device Feature Header (DFHv1).

Signed-off-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20230115151447.1353428-2-matthew.gerlach@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>