dt-bindings: pwm: intel,keembay: 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>
Acked-by: Vijayakannan Ayyathurai <vijayakannan.ayyathurai@intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>