dt-bindings: Add syscon YAML description

The syscon binding is a pretty loose one, with everyone having a bunch of
vendor specific compatibles.

In order to start the effort to describe them using YAML, let's create a
binding that tolerates additional, not listed, compatibles.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>