pwm: kona: Introduce Kona PWM controller support

Add support for the six-channel Kona PWM controller found on Broadcom
mobile SoCs like bcm281xx.

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>