dt-bindings: input: gpio-keys: Fix interrupts in example

The "interrupts" property in the example looks weird:
  - The type is not in the last cell,
  - Level interrupts don't work well with gpio-keys, as they keep the
    interrupt asserted as long as the key is pressed, causing an
    interrupt storm.

Use a more realistic falling-edge interrupt instead.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/47ecd2d8efcf09f8ab47de87a7bcfafc82208776.1638538079.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>