dt-bindings: serial: sh-sci: Convert to json-schema

Convert the Renesas Serial Communication Interface ((H)SCI(F)(A|B))
Device Tree binding documentation to json-schema.

Split the bindings in 5 files, one per major type, to ease expressing
constraints.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>