avr32: don't offer CONFIG_RTC

This patch fixes the following compile error:

<--  snip  -->

...
  CC [M]  drivers/char/rtc.o
In file included from /home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/char/rtc.c:70:
/home/bunk/linux/kernel-2.6/git/linux-2.6/include/linux/mc146818rtc.h:16:59: error: asm/mc146818rtc.h: No such file or directory
...
make[3]: *** [drivers/char/rtc.o] Error 1

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>