ARM: dts: Declare the PWM for bcm11351 (bcm281xx)

Add the device tree node for the PWM on bcm11351 SoCs.

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>