pwm: renesas-tpu: Add DT support

Specify DT bindings for the TPU PWM controller and add OF support to the
driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>