dt-bindings: pwm: Add R-Car D3 device tree bindings

Add device tree bindings for the PWM controller found on R-Car D3 SoCs.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>