pwm: add DT bindings documentation for atmel-hlcdc-pwm driver

The HLCDC IP available in some Atmel SoCs (i.e. at91sam9x5, at91sam9n12
or sama5d3 families for instance) provides a PWM device.

The DT bindings used for this PWM device is following the default 3 cells
bindings described in Documentation/devicetree/bindings/pwm/pwm.txt.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>