ARM: dts: Enable the PWM for bcm28155 AP board

Mark the PWM as enabled on the bcm28155 AP board.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>