doc: dt: vendor-prefixes: Add Digilent Inc

Digilent is a board designer, making various Linux capabable FPGA and
processor boards. Add to the vendor list.
Acked-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -40,6 +40,7 @@ dallas	Maxim Integrated Products (formerly Dallas Semiconductor)
 davicom	DAVICOM Semiconductor, Inc.
 denx	Denx Software Engineering
 digi	Digi International Inc.
+digilent	Diglent, Inc.
 dlg	Dialog Semiconductor
 dlink	D-Link Corporation
 dmo	Data Modul AG