ARM: pxa: pxa27x.c: add dummy SA1100 rtc clock

Using pxa27x you could now build both RTC_DRV_PXA and RTC_DRV_SA1100.
Make sure you don't use both together: link /dev/rtc0 or /dev/rtc1
to /dev/rtc according to your requirement.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>