dt-bindings: vendor-prefixes: Add U-Boot bootloader prefix

List U-Boot project in vendor prefixes.

For more information take a look at:
https://en.wikipedia.org/wiki/Das_U-Boot
Source code is available here:
https://gitlab.denx.de/u-boot/u-boot



Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>