gpiopower: trigger low, high and both edges

Power off or reset may be activated either by low or high signal or by an
edge. So, try everything.

Also, the driver now supports DTS properties for timings.

Finally, the driver does not change the pin configuration during attach.
It is assumed that the pin is already in a state that does not trigger
the power event (otherwise we wouldn't be running).

(cherry picked from commit 320e4beb97618c6964380bfa404a3e96c5de7663)