dt-bindings: i3c: Document core bindings

A new I3C subsystem has been added and a generic description has been
created to represent the I3C bus and the devices connected on it.

Document this generic representation.

Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>