tty: serial: fsl_lpuart: Use cpu_relax() instead of barrier()

Use cpu_relax() instead of barrier() in a tight polling loops to make
them a bit more idiomatic. Should also improve things on ARM64 a bit
since cpu_relax() will expand into "yield" instruction there.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Cory Tusar <cory.tusar@zii.aero>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-imx@nxp.com
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20190729195226.8862-16-andrew.smirnov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -553,14 +553,14 @@ static void lpuart_wait_bit_set(struct uart_port *port, unsigned int offset,
 				u8 bit)
 {
 	while (!(readb(port->membase + offset) & bit))
-		barrier();
+		cpu_relax();
 }
 
 static void lpuart32_wait_bit_set(struct uart_port *port, unsigned int offset,
 				  u32 bit)
 {
 	while (!(lpuart32_read(port, offset) & bit))
-		barrier();
+		cpu_relax();
 }
 
 #if defined(CONFIG_CONSOLE_POLL)