dt-bindings: serial: Convert slave-device bindings to json-schema

Convert the serial slave-device Device Tree binding documentation to
json-schema, and incorporate it into the generic serial bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Rob Herring <robh@kernel.org>