dt-bindings: fpga: xilinx-slave-serial: valid for the 7 Series too

The Xilinx 7-series uses the same protocol, mention that.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>