dt-bindings: arm: Document Actions Semi S500

The Actions Semi S500 is a quad-core ARM Cortex-A9 SoC.
The LeMaker Guitar is an SODIMM-format module with that SoC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>