hw/char: QOM'ify cadence_uart model

* drop qemu_char_get_next_serial and use chardev prop
* create cadence_uart_create wrapper function to create
  cadence_uart_device
* change affected board code to use the new way

Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-id: 1465028065-5855-3-git-send-email-zxq_yx_007@163.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>