dt-bindings: power: Fix "debounce-interval" property misspelling

"debounce_interval" was never supported.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>