ARM: dts: imx51-zii-rdu1: Fix the rtc compatible string

According to Documentation/devicetree/bindings/rtc/rtc-ds1307.txt the
original compatible "maxim,ds1341" is not a valid entry.

Switch to the documented "dallas,ds1341" compatible.

Reported-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>