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>

Documentation/devicetree/bindings/fpga/xilinx-slave-serial.txt

 Xilinx Slave Serial SPI FPGA Manager
 
-Xilinx Spartan-6 FPGAs support a method of loading the bitstream over
-what is referred to as "slave serial" interface.
+Xilinx Spartan-6 and 7 Series FPGAs support a method of loading the
+bitstream over what is referred to as "slave serial" interface.
 The slave serial link is not technically SPI, and might require extra
 circuits in order to play nicely with other SPI slaves on the same bus.
 
-See https://www.xilinx.com/support/documentation/user_guides/ug380.pdf
+See:
+- https://www.xilinx.com/support/documentation/user_guides/ug380.pdf
+- https://www.xilinx.com/support/documentation/user_guides/ug470_7Series_Config.pdf
+- https://www.xilinx.com/support/documentation/application_notes/xapp583-fpga-configuration.pdf
 
 Required properties:
 - compatible: should contain "xlnx,fpga-slave-serial"