hw/arm/pxa2xx_gpio: Correct and register vmstate

hw/arm/pxa2xx_gpio: Correct and register vmstate

The pxa2xx-gpio device has a VMStateDescription, but it was accidentally
never actually registered, and it wasn't quite correct. Remove the
'lines' field (this is a device property, not mutable state), add the
missing 'prev_level' field, and set dc->vmsd so it actually gets used.

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