The lpuart of imx8ulp is basically the same as imx7ulp, but it supports
some new features based on imx7ulp, such as it can assert the DMA
request on EOP(end-of-packet).

Here add lpuart support for imx8ulp, and rx_watermark is set to 3 as
imx8ulp RX FIFO depth is 8.
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20230130064449.9564-7-sherry.sun@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>