pwm: atmel-pwm: Add Atmel PWM controller driver

Add a PWM framework driver for the PWM controller found on Atmel SoCs.

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>
[thierry.reding: coding style and other minor cleanups]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>