rtc: x1205: Add DT bindings

This adds device tree bindings for the Xircom X1205 RTC found
in the Linksys NSLU2.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[alexandre.belloni@bootlin.com: move doc to rtc.txt]
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>