avr32: atmel_default_console_device depends on CONFIG_SERIAL_ATMEL

allnoconfig:

arch/avr32/mach-at32ap/built-in.o: In function `at32_select_gpio':
(.init.text+0x548): undefined reference to `atmel_default_console_device'
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>

arch/avr32/mach-at32ap/at32ap700x.c

@@ -1060,7 +1060,9 @@ struct platform_device *__init at32_add_device_usart(unsigned int id)
 
 void __init at32_setup_serial_console(unsigned int usart_id)
 {
+#ifdef CONFIG_SERIAL_ATMEL
 	atmel_default_console_device = at32_usarts[usart_id];
+#endif
 }
 
 /* --------------------------------------------------------------------