efi_loader: fill simple network protocol revision

Provide the simple network protocol revision.
This revision number could be used to identify backwards compatible
enhancements of the protocol.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>