of: Add Atmel PWM controller device tree binding

The Atmel PWM controller uses the standard device tree bindings for PWM
controllers.

Signed-off-by: Bo Shen <voice.shen@atmel.com>

Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>