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>