dt-bindings: pwm: Document Synopsys DesignWare snps,pwm-dw-apb-timers-pwm2

Add documentation for the bindings for Synopsys' DesignWare PWM block
as we will be adding DT/platform support to the Linux driver soon.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>