[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>