dt-bindings: arm: Add Calxeda system registers json-schema binding

The Calxeda system registers are a collection of MMIO register
controlling several more general aspects of the SoC.
Beside for some power management tasks this node is also somewhat
abused as the container for the clock nodes.

Add a binding in DT schema format using json-schema.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>