dt-bindings: arm: Document compatibles for Ux500 boards

The device-specific compatible values used by the Ux500 boards
were not documented so far. Add a new simple schema to document them.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20191120181857.97174-4-stephan@gerhold.net


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