dt-bindings: pwm: renesas: tpu: convert bindings to json-schema

Convert Renesas R-Car Timer Pulse Unit PWM Controller bindings documentation
to json-schema.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>