at91rm9200: move serial shutdown code to serial drivers

introduce serial_exit for this purpose. Use it only when the rm9200
serial driver is active

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

include/common.h

@@ -453,6 +453,7 @@ void ft_pci_setup(void *blob, bd_t *bd);
 
 /* $(CPU)/serial.c */
 int	serial_init   (void);
+void	serial_exit   (void);
 void	serial_addr   (unsigned int);
 void	serial_setbrg (void);
 void	serial_putc   (const char);