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

Convert device tree bindings for SiFive's PWM controller to YAML
format.

Signed-off-by: Sagar Kadam <sagar.kadam@sifive.com>
Link: https://lore.kernel.org/r/1601393531-2402-4-git-send-email-sagar.kadam@sifive.com


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