overlays: gpio-shutdown: Add debounce parameter

Give the gpio-shutdown overlay a debounce parameter that requires
the GPIO to be held at the relevant level for a specified number
of milliseconds. The default value is 100 - higher than the driver
default of 5ms to avoid unfortunate glitches.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=253680



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