Support for UDOO Quad/Dual power button

[graugans]

The Quad/Dual boards are equipped with a PWR button. when shutting down the board with

$ halt

The board wakes up again but during runtime nothing happens when pressing this button.

Some details from the i.mx6 hardware dev guide

There is a driver for snvs-pwrkey but seems not activated by device tree.

[graugans]

Here is a workaround for the issue: http://www.clarenceho.net/2016/05/adding-shutdown-button-to-udoo.html?m=1

[modjo756]

I think to add the pwr button working, need to add this during the kernel build : power