ARM: dts: ux500: Add devicetree for HREF520

This reference design is very similar to the others just that
it has a different display mounted on the user interface
board, and some GPIOs where shuffled around.

As this is the first board that uses DB8520 we create the
DB8520-specific DTSI file here.

Cc: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191126124738.77690-3-linus.walleij@linaro.org