tty: serial: fsl_lpuart: Enable Receiver Idle Empty function for LPUART

tty: serial: fsl_lpuart: Enable Receiver Idle Empty function for LPUART

With the growth of rx watermark, it's useful to enable the Receiver Idle
Empty function, it can assert the RDRF(Receive Data Register Full Flag)
when the receiver is idle for a number of idle characters and the FIFO
is not empty. It will generate a DMA request or interrupt, which can
avoid receive data being trapped in the RX FIFO since the number of
words received is less than the watermark.

Here set the RXIDEN as 0x3 which enable the RDRF assertion due to
partially filled FIFO when receiver is idle for 4 characters.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20230130064449.9564-5-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>