hw/arm/fsl-imx*: Don't create "null" chardevs for serial devices

hw/arm/fsl-imx*: Don't create "null" chardevs for serial devices

Following commit 12051d82f004024, UART devices should handle
being passed a NULL pointer chardev, so we don't need to
create "null" backends in board code. Remove the code that
does this and updates serial_hds[].

(fsl-imx7.c was already written this way.)

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-id: 20180420145249[email protected]