rtc: x1205: Add DT probing support

This makes it possible to probe the X1205 RTC from the
device tree. This is needed when adding device tree boot
support for the IXP4xx-based NSLU2 which has this RTC.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>