[ARM] Fix ateb9200_defconfig build regression

Fix:

  CC      drivers/serial/atmel_serial.o
drivers/serial/atmel_serial.c: In function 'atmel_serial_suspend':
drivers/serial/atmel_serial.c:924: error: implicit declaration of function 'at91_suspend_entering_slow_clock'
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

include/asm-arm/arch-at91/board.h

@@ -130,4 +130,7 @@ extern u8 at91_leds_cpu;
 extern u8 at91_leds_timer;
 extern void __init at91_init_leds(u8 cpu_led, u8 timer_led);
 
+/* FIXME: this needs a better location, but gets stuff building again */
+extern int at91_suspend_entering_slow_clock(void);
+
 #endif