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.

drivers/power/reset/gpio-poweroff.c

@@ -49,9 +49,11 @@ static int gpio_poweroff_probe(struct platform_device *pdev)
 {
 	bool input = false;
 	enum gpiod_flags flags;
+	bool force = false;
 
 	/* If a pm_power_off function has already been added, leave it alone */
-	if (pm_power_off != NULL) {
+	force = of_property_read_bool(pdev->dev.of_node, "force");
+	if (!force && (pm_power_off != NULL)) {
 		dev_err(&pdev->dev,
 			"%s: pm_power_off function already registered",
 		       __func__);