dt-bindings: pwm: Convert atmel pwm to json-schema

Convert PWM binding for Atmel/Microchip SoCs to Device Tree Schema
format.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>