dt-bindings: Add Xylon vendor prefix

Xylon is an electronics company that produces FPGA hardware block designs
optimized for Xilinx FPGAs.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191203141243.251058-2-paul.kocialkowski@bootlin.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>