hw/xtensa/xtfpga.c: Don't create "null" chardevs for serial devices

hw/xtensa/xtfpga.c: 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[].

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-6-peter.maydell@linaro.org