Avoid to hard coded irq in rtc-sa1100 driver since we could share
it among arch-sa1100/arch-pxa/arch-mmp.

We still keep hard coded register address since the requirement is
enabling both rtc-sa1100 and rtc-pxa driver. The register addresses
are conflict since they're only two wrappers on the same rtc device.
Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>