Implement named GPIOs on the Device layer. Listifies the existing GPIOs
stuff using string keys. Legacy un-named GPIOs are preserved by using
a NULL name string - they are just a single matchable element in the
name list.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>