power button: long press doesn't turn on devkit

Feature: https://source.puri.sm/Librem5/use-cases/issues/87

I would expect that a long press of the power button on the devkit would turn off the devkit and then another long press would turn it back on. Currently, a long press will turn off the devkit correctly, but another long press does nothing to turn on the devkit. @angus.ainslie suggested that this is likely due to the pmic being in the wrong state at power off.

Instead, to turn on the devkit, the usb cable needs to be unplugged and replugged into the workstation.