tty: serial: fsl_lpuart: make rx_watermark configurable for different platforms

Add rx_watermark parameter for struct lpuart_port to make the receive
watermark configurable for different platforms.
No function changed.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20230130064449.9564-2-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>