nand: Don't inherit from Sysbus

Nand chips are not sysbus devices - they do not have any sense of MMIO,
nor interrupts. Re-parent to TYPE_DEVICE accordingly.

Cc: afaerber@suse.de

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