Documentation: dt: Add Kona PWM binding

Add the binding description for the Kona PWM controller found on Broadcom's
mobile 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: Thierry Reding <thierry.reding@gmail.com>