dt-bindings: power: reset: gpio-poweroff: Add 'timeout-ms' property

Add 'timeout-ms' property to support boards where the 3s timeout that the
current driver defaults to is too short.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>