Re-implement the Sysbus GPIOs to use the existing TYPE_DEVICE
GPIO named framework. A constant string name is chosen to avoid
conflicts with existing unnamed GPIOs.

This unifies GPIOs are IRQs for sysbus devices and allows removal
of all Sysbus state for GPIOs.

Any existing and future-added functionality for GPIOs is now
also available for sysbus IRQs.

Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>