gpio-poweroff: Remember the old poweroff handler

Keeping a copy of the old poweroff handler allows it to be restored
should this module be unloaded, but also provides a fallback if the
power hasn't been removed when the timeout elapses.

See: https://github.com/raspberrypi/rpi-eeprom/issues/330



Signed-off-by: Phil Elwell <phil@raspberrypi.com>