gpio-poweroff: Allow it to work on Raspberry Pi

The Raspberry Pi firmware manages the power-down and reboot
process. To do this it installs a pm_power_off handler, causing
the gpio-poweroff module to abort the probe function.

This patch introduces a "force" DT property that overrides that
behaviour, and also adds a DT overlay to enable and control it.

Note that running in an active-low configuration (DT parameter
"active_low") requires a custom dt-blob.bin and probably won't
allow a reboot without switching off, so an external inversion
of the trigger signal may be preferable.