dt-bindings: riscv: convert plic bindings to json-schema

Convert device tree bindings for SiFive's PLIC to YAML format

Signed-off-by: Sagar Kadam <sagar.kadam@sifive.com>
Link: https://lore.kernel.org/r/1601393531-2402-3-git-send-email-sagar.kadam@sifive.com


Signed-off-by: Rob Herring <robh@kernel.org>