ARM: pxa: add dummy clock for sa1100-rtc

Now sa1100-rtc can support sa1100/pxa/mmp soc series, then we need
add dummy clock for sa1100-rtc.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>