dt-bindings: pwm: renesas,tpu: Do not require pwm-cells twice

pwm-cells property is already required by pwm.yaml schema.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>