ARM: dts: imx6ul: peb-av-02: move to 3 cell pwm

Instead of changing default pwm-cells property, use the default
"#pwm-cells = <3>" and add the third option.

Signed-off-by: Andrej Picej <andrej.picej@norik.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>