dt-bindings: arm: Document Actions Semi S900

The Actions Semi S900 is a quad-core ARM Cortex-A53 SoC.
The Bubblegum-96 is a 96Boards Consumer Edition compliant board (4/96).

Cc: 96boards@ucrobotics.com
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>