ARM: dts: ux500: Add "simple-bus" compatible to soc node

ARM: dts: ux500: Add "simple-bus" compatible to soc node

The "soc" node in the Ux500 device tree does not need any special
handling - it is just a simple I/O bus that can be accessed without
additional configuration.

Therefore we can additionally describe it as compatible with "simple-bus".
This can be used by platforms to probe devices under the soc node without
special handling for our custom "stericsson,db8500" compatible
(e.g. in U-Boot).

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20191124195728.32226-1-stephan@gerhold.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>