ARM: dts: imx6ul: disable PWMs by default

Since PWMs are only useful if they are actually connected to an output pin,
let users enable them explicitly in their device trees where they should
also set up the pin configuration. This is in sync with a recent change
(commit e2675266b39b ("ARM: dts: imx6qdl: disable PWMs by default"))
to other i.MX SoCs.