ARM: pxa: add dummy SA1100 rtc clock in pxa25x

* fix failure seen on Zaurus corgi:
* sa1100-rtc sa1100-rtc: failed to find rtc clock source
* sa1100-rtc: probe of sa1100-rtc failed with error -2

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>