Convert Broadcom Kona family PWM controller bindings to DT schema.

Change during conversion:
  - add used, but previously undocumented brcm,bcm11351-pwm compatible
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>