rtc: sa1100: prepare to share sa1100_rtc_ops

Factor out the RTC initialization from the platform device specific
parts in order to share the RTC device ops with other drivers.
Specifically, it will be shared with rtc-pxa driver.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: rtc-linux@googlegroups.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>