machine: Move gpio code to hw/core/gpio.c

Only softmmu code uses gpio, so move gpio code from qdev.c to
gpio.c and compile it only on softmmu mode.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190425200051.19906-2-ehabkost@redhat.com>
[PMD: Rebased]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>