pwm: Add device tree support

This patch adds helpers to support device tree bindings for the generic
PWM API. Device tree binding documentation for PWM controllers is also
provided.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>