overlays: gpio-keys: Avoid open-drain warnings

The i2c-gpio driver expects to use a GPIO in open-drain mode. Failure
to configure it in that way causes alarming warnings in the kernel log.
The BCM283x and BCM2711 GPIO blocks don't support open-drain mode,
but i2c-gpio works anyway. Silence the warning by declaring that
open-drain mode has been enabled by other means.

See: https://github.com/raspberrypi/firmware/issues/1381



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