Documentation: Add device tree bindings for Freescale FTM PWM.

This adds the binding documentation for Freescale FlexTimer Module
(FTM) PWM driver under Documentation/devicetree/bindings/pwm/.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Yuan Yao <yao.yuan@freescale.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>