tty: serial: 8250: 8250_port: Move prototypes to shared location

Fixes the following W=1 kernel build warning(s):

drivers/tty/serial/8250/8250_port.c:349:14: warning: no previous prototype for ‘au_serial_in’ [-Wmissing-prototypes]
drivers/tty/serial/8250/8250_port.c:359:6: warning: no previous prototype for ‘au_serial_out’ [-Wmissing-prototypes]

Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Cc: Mike Hudson <[e-mail]>
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_early.c

···
 
 #ifdef CONFIG_SERIAL_8250_RT288X
 
-unsigned int au_serial_in(struct uart_port *p, int offset);
-void au_serial_out(struct uart_port *p, int offset, int value);
-
 static int __init early_au_setup(struct earlycon_device *dev, const char *opt)
 {
 	dev->port.serial_in = au_serial_in;

include/linux/serial_8250.h

···
 					(int port, struct uart_port *up,
 						u32 *capabilities));
 
+#ifdef CONFIG_SERIAL_8250_RT288X
+unsigned int au_serial_in(struct uart_port *p, int offset);
+void au_serial_out(struct uart_port *p, int offset, int value);
+#endif
+
 #endif