overlays: Clarify gpio-poweroff semantics in README

1) GPIO pin is signalled on poweroff, not reboot.
2) In RPi kernels, halt is equivalent to poweroff.

See: #1600

Author: Phil Elwell

arch/arm/boot/dts/overlays/README

@@ -340,7 +340,7 @@ Params: gpio_pin                Input pin number. Default is 18.
 
 
 Name:   gpio-poweroff
-Info:   Drives a GPIO high or low on reboot
+Info:   Drives a GPIO high or low on poweroff (including halt)
 Load:   dtoverlay=gpio-poweroff,<param>=<val>
 Params: gpiopin                 GPIO for signalling (default 26)