MLK-16896 tty: serial: lpuart: flush transmit/receive fifo/buffer

MLK-16896 tty: serial: lpuart: flush transmit/receive fifo/buffer

Although .startup() alreadly do transmit/receive fifo/buffer flush,
but switch the baud rate may introduce dirty data on fifo, in such
case, user will call tcflush() to clean up buffer and fifo. So driver
also ensure HW fifo is cleaned up.

The patch add hw fifo/buffer flush in .flush_buffer() callback.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>