arm: ls1021atwr: Enable driver model lpuart serial driver

Convert ls1021atwr_nor_lpuart to driver model support. As a start,
enable lpuart serial port driver.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alison Wang <alison.wang@nxp.com>
Tested-by: Alison Wang <alison.wang@nxp.com>
Acked-by: Simon Glass <sjg@chromium.org>