dt-bindings: serial: Convert riscv,sifive-serial to json-schema

Convert the riscv,sifive-serial binding to DT schema using json-schema.

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1567592383-8920-1-git-send-email-pragnesh.patel@sifive.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>