dt-bindings: pwm: ti,pwm-omap-dmtimer: Update binding for yaml

Update for yaml and remove the old txt binding.

As we can replace most of the custom timer API with standard Linux
frameworks such as clock framework, let's tag the properties for
ti,prescaler and ti,clock-source as deprecated.

Cc:  <linux-pwm@vger.kernel.org>
Cc: Nishanth Menon <nm@ti.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>