qdev: gpio: Register GPIO outputs as QOM links

Within the object that contains the GPIO output. This allows for
connecting GPIO outputs via setting of a Link property.

Also clear the link value to zero. This catch-alls the case
where a device improperly inits a gpio_out (malloc instead of
malloc0).

Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
